The reason I was so standoffish in the matches is because Nainsoo is pretty good at punishing almost any of my approaches, especially if my spacing is off. So I try and punish his instead (I don't do this very well though). I also am not very familiar with the peach matchup, except for avoiding d-smash at all costs and if possible punishing it. I just don't know how to approach her and get grabs and combos started without the fear of getting punished by her high priority moveset ( in general). I felt like most of the grabs I got during the match were simply because he made an unforced mistake.
A big part of Sheik vs Peach is trolling her with needles to make her jump or want to approach and then you punish either accordingly. A lot of spacing goes into this MU and you really have to pick your moves carefully based on how you're positioned in relation to Peach.
The rest of it is Bair, to a lesser degree Fair, to an even lesser degree other moves, and how you get grabs and do throw follows.
Grabs will usually be accumulated as a result of putting her on the defensive and making her sweat in some way. The obvious one is blocking... you'll pressure her into block and then either grab her right there, or bait a roll/sidestep (Peaches love their sidesteps) and then grab her in its lag.
The other ways grabs happen when you pressure her is that you'll make her feel all tense and desperate and she'll try to dash attack or D-smash (or other stuff, but those are most common) to try and regain control by force and then you'll space around those (or block the dash attack) and grab her in the lag.
If she tries to be too sneaky with her shield pressure then grabbing is works. But I wouldn't recommend trying to shield grab Peach unless it's really obvious you can. Her pressure is really good.
Also, I don't know how to reliably accumulate damage early in a stock against Peach.
This will help you learn how to rack damage on Peach.
Otherwise, it's similar to comboing Marth only with more narrow percents. If she DIs away from your throw you can regrab to 20. After that you have to do stuff. If she DIs anywhere else below about 10, you can link U-tilt and then follow with Uair or Fair depending on how you want to follow her. After about 11 or so, you have to switch to Uair.
At about 60 or so, if she's not DIing the throw, you can stop Uairing if you want and focus on hitting her offstage (I forget why but I think she starts going too far for her to be really effectively chased... though Sheik's range still lets her cover a bunch of options). If you can force her onto the ledge, you can troll her with needles and spaced aerials / grabs to make it difficult for her to get back on the stage.
At about 64 or so (to some percent, I forget what), if she's not DIing the throw away and there's no platform interference, you can try to combo a tipped U-smash and get the KO. Doesn't always work, though.
Something that might help you:
To learn how to max range your moves more effectively there are some very easy things you can do.
Go to versus mode, set time to infinite (or whatever). Open two players. You will be one of them. The other will not be playing (you just need a 2nd controller). From there, set your handicap to 1 and leave the empty at 9. Make the empty be someone big. I personally recommend Bowser or DK.
From there, just practice hitting and grabbing from as far away as you can, and get used to moving into and out of those ranges. It's very good and helps you look for visual cues for when you're in range or not. Use a variety of aerials at different heights... her Fair's ability to hit horizontally changes a bit depending on if you do it high enough to autocancel, or low enough to need an L-cancel.
These sorts of drills help my tech skill a lot lolol. Pick a level like Battlefield or FoD so you can see how to waveland into these moves. Dreamland can be annoying 'cuz of wind.
If you lack a 2nd controller, use a level 1 Bowser.
