I'm a Marth vs. Falco beast, so I'll drop some knowledge on common things.
Lasers:
Lasers are mad important in this matchup, but important doesn't mean to spam. You usually will want to stick with only 1-2 lasers in a row before mixing in a dash dance. Marths love to use power shield gimmicks (like power shield fsmash or WD OoS grab), and even if they don't power shield they often aren't afraid to just trade an fsmash with a laser. When you are lasering, you have to have hyper-acuity of how close they are to tipper range. If you are within tipper range, you absolutely MUST be approaching them hard. Dash dancing or lasering within WD -> tipper range is bad enough, but you can at least react if they try to WD in on you. Lasers should basically be serving as your spacing tool to make sure they can not maintain this tipper spacing. Dashing back with a quick reverse laser can frequently get you out of his tipper range, but obviously you can't dash backwards all day because you'll get cornered. Instead you will either want to approach immediately after connecting with the reverse laser or try and go above them onto the top plat. You may also want to mix in one or two more lasers after the reverse laser just so they don't start anticipating you jumping above them or approaching immediately. On an overall note, your laser heights are very important to keep track of. Low lasers will prevent him from dash dancing under them, but every once in a while when you expect them to approach with a SHFFL you should pop out a quick high laser. Lasering Marth out of a SHFFL is just a free nair approach, or even a laser-fsmash kill at higher percents. These high lasers also apply to a Marth drifting downward from a recovery or combo on stage. A last thing to be careful about is getting dash attacked when you retreating laser. Do not fall for this because dash attack tends to lead to huge combos for Marth. Mixing in dash dances between lasers helps this a lot because they can't rely on you to be lasering every time you dash back. As soon as you see them going for it, a simple SHFFL will usually shut it down. Marth landing a dash attack is almost entirely prediction based, meaning they will only land it if you dash back and laser when they think you will.
The Approach:
Branching from the lasers section is appropriate because lasers are extremely helpful for setting up a safe approach, but you should NOT rely on them to approach. It is tempting to want to just SHL 2-3 times as you run towards them and aerial, but it will almost never work. Instead, like I said above, mixing in the dash dance is one of the most important elements of the approach.
PP does this incredibly in his sets with Mew2King at RoM3 and Pound 5. Marth can throw out fairs, utilts, WD back grab, counter, and use his huge dash dance to completely destroy any approach that isn't perfect. Using a laser or two followed by a dash dance can absolutely CRIPPLE a Marth who relies the Falco to run into these attacks/traps (you'd be surprised how often even good Falco players do). The dash dance baits all of those, and then it is simply a matter of reacting accordingly. If they throw out an attack, obviously you can just rush in and punish the lag. Marth's attacks come out pretty quickly, but most of them do not have lingering hitboxes which means rushing in on reaction is possible. If they WD back or dash dance back, it means they are less hesitant to assume you will approach, which is why this is much more common among higher skilled Marths. The way I view this is just them giving up space.
Just because you are dash dancing to bait a reaction does not mean you must approach as soon as they do something. Many players will actually take a dash dance as a sign that you ARE approaching, so make sure you mix in lasers before as well as after your dash dances to keep them guessing. This way if they expect the approach and are WDing/dash dancing you can just laser them to throw them off and approach from there, or dash dance to bait another reaction. Because of lasers, I like to think of neutral in this matchup as almost being in Falco's favor, but it often depends on how aggressive the Marth is (I believe aggressive Marths can shut down Falco's neutral game, but it is definitely difficult and more risky than just playing passively and waiting for them to make a mistake). So unless the Marth decides to approach, you have almost full control, and you just have to be patient with your laser dash dance game, approaching only when you are confident you can do safely.
The Pressure:
So you've outspaced Marth and are committing to an approach. Now what? A first thing that can make or break your approach is the actual spacing of your attack. If you go in for a laser, you have to make sure you can reach him with a shine or grab. Jab can work, but Marth can usually aerial you out of it and it's really just asking to get shield grabbed. Most often, you'll want to approach with a nair or dair. I typically go for a dair if I think I can get the hit because it has a higher reward on punishment, but if I think at all they may shield I will nair. You have to be careful to overshoot your aerials to cover those dash dance back and WD back options, but you also can't overdo the spacing because Marth CAN dash under you, which can **** you pretty hard if he is able to reverse the situation and pin you by the ledge. So space your aerial, and also make sure you won't get grabbed by using a late aerial-shine. From here, you enter shield mixup games.
If they are 70%ish or above, I love just shine grabbing and uthrow dairing them. The only way out of it is a spot dodge which will get them destroyed if you do anything other than grab. If you aren't grabbing, the number one thing you want to watch for is the WD OoS. This is by far Marth's strongest OoS option, and sloppy shield pressure will get you hit by WD OoS fsmash ALL DAY by a Marth who knows what he's doing. Even proper shield pressure leaves frames where they can WD OoS, and Marth can usually slip through almost anything with his WD because of how far it goes. This means even if he is pinned at the ledge he could easily cross you up with a WD OoS, and that often leads to your death. I prefer to time my pressure so that they have to WD after the shine. This makes it so you can do that pseudo-reacting/predicting combination to reverse SHL out of your shine. Mango uses this flawlessly in his match with
Mew2King at Genesis. When you laser their WD OoS (or roll for that matter), you can immediately follow up with another aerial-shine to keep the pressure on them. Marth's shield is pretty horrible, so after 2 iterations of aerial-shine pressure, I am almost always going for a shield poke to combo off of. The other main option to cover their OoS stuff is an instant fade away aerial. It's very effective at baiting grabs, and it doubles as a cover for WD OoS if they are trapped at the ledge and try to cross you up (
Ahhh, ****).
The Punishment:
Falco combos the balls off of Marth, so it's important to make every hit worth your while. One of the most frustrating parts of this matchup is it feels like it takes ages for you to even get a hit, and there's nothing more frustrating then dropping a combo once you do. If you grab at low percent, uthrow fair ends up doing 30%+, so it is a reliable damage builder, and used near the ledge it can do a decent job of forcing an edge guard (they will want to DI in so you are almost always guaranteed 30% near the ledge). Most of the time, however, you will be punishing after landing a shine. Falco's classic double jump dair pillar works spectacularly on Marth. I suggest practicing on FD to get this combo down. After you dair, you have to fast fall to make sure you get to the ground first, and from there you will either shine again or utilt. There isn't a large difference between whichever, but I don't like staling my shine so I will usually only shine when they are at low %s and they are close to the ground so I can get them quickly. Utilt tends to be easier to hit, and once they've been pillared to a high enough percent, the dair will send them into the ground. If you are going to drop a combo, this is usually when it happens. On FD I tend to just tech chase them with a laser into aerial (or aerial-shine if they get their shield up in time). Dair is usually best at this portion of the stock because they will be at good %s to double dair spike or dair to bair/uair. Nair will get them off stage, but unless they were really high (like 150%), you'll risk dropping the edge guard.
On all of the other stages, you'll typically be dairing them onto platforms. If they are still pretty low percent, you can continue shining/utilting them before they hit the ground just like you would on FD (though it's harder because the distance is smaller). As soon as they reach the percent where they will land on the platform before you can hit them, you just need to react and punish accordingly. Expect them to miss the tech or tech in place. You can dsmash both on reaction, or if they are high enough to kill with a dtilt that is better because it takes ground teching out of the equation. Even if they do ground tech, however, you should be able to just dsmash again/fsmash (if they tech rolled) or lead into an edge guard (if they teched in place and slid off). Because of the tech in place slide-off, I try to always be on the inside portion of the platform so that it doesn't send them sliding in the other direction where they could live. If they DON'T miss the tech or tech in place, they have tech rolled, which you can react to for a free fsmash. If you ever get into this tech situation when they are not at a high enough percent to KO, you can always punish with a longer string to tack on a bit more damage before sending them off stage. Utilt -> uair/bair, double dair spike, or even a grab into uthrow uair/dair/bair will effectively punish Marth, getting him decently far off stage, if not KO'd.
The Edgeguard:
A lot of people let Marth grab the ledge, but this doesn't cut it in close matches when you need every combo to lead to a KO. The main way you will be dealing with Marth's off stage side-B, fair, and up-B is to simply learn to abuse the ledge invincibility. It's almost helpful to think up a list of rules that you will follow to consistently edge guard Marth no matter what. Mine would be something like:
1. If Marth up-Bs when he CANNOT make it back to the stage, roll from the ledge. (simple enough)
2. If Marth up-Bs when he CAN make it back to the stage (though might still go for the ledge), wait on the ledge and LH an attack to send him back off. If you mistimed grabbing the ledge for invincibility and cannot wait on the ledge to react, regular get-up into dsmash will usually work, or if he is pretty close to the stage so that his up-B would land him far on stage, you can roll from the ledge and fsmash.
3. If you LH an attack from rule 2, it should be a dair or bair. You can double dair spike them until they start DI'ing away from the stage to prevent it, at which point you can bair them and kill them at low percents. It becomes very RPS like, but you can typically cover all of the options regardless of whether or not you guess right. Even if they guess the dair/bair and DI correctly, they will still be far enough off stage where you can simply "reset" the edge guarding mini game and repeat.
4. If Marth is high enough that he can jump onto the ledge or stage, simply grab the ledge for invincibility and drop into a quick rising bair or shine bair off the ledge.
5. In a situation where you cannot get to the ledge quickly enough to do any of the above options, walk up dsmash will connect (even through a large portion of the ledge because of Marth's up-B hurtbox).
That is mostly just the basics for edgeguarding Marth. There is a lot of nuance for certain situations (such as reacting to their ledge techs or the best way to get invincibility without getting hit by up-B), but most of it becomes obvious with some practice and experience, and I would be typing all day if I decided to explain it all. Hopefully I helped someone. I honestly didn't plan on typing this much when I started. lol