Learning to fast fall when using your aerials helps a lot with Ness. Like full hop bair then bair again, but after full hop bair and fast fall the bair to get an idea on your speed difference.
Ness' magnet while overall meh does let him cancel his 2nd jump and get towards the ground faster so it has a 2nd use as well. Why we can't just have double jump cancels + rising double jumps like PM/brawl+/and so on is a mystery really.
Of course one more thing to figure out is how to buff your 2nd jumps' range. Fair + double jump at the same time buffs the horizontal range a bit of it some. Timing your air dodge + 2nd jump at the right time lets you buff the vertical and horizontal range of it. Both can help with recovery.
Also learning Ness' magic range is a good thing in game. You never want to be too close until you are punishing people and dishing out pain. You for sure don't want to keep your distance from people cause Ness has overall slow speed, **** attack range, and chasing after people is not a good thing for him because of it.
Keeping people within range of PK Fire (but not actually using it much, just keeping in reach) let you get better at punishing mistakes. If you have someone running from you/keeping you out throw out PKT1 to attempt to tag them every now n then. Don't spam it cause PKT in general leaves you open/kills u on mistake but PKT is your best bet to bring people into your wanted attack ranges.
Ness has pretty low movement speeds in general and while his moves are pretty fast they aren't lightning fast. I mean Ness' nair is frame 5 but other characters have even faster nairs like Mario, Yoshi, Luigi, Lucas, Villager. So making people not run from you or be flat out in your face is a good thing to figure out how to do.
Ness can set up some stuff with footstool. On ground he can pop some characters up for a combo kill with PKT2 even. And pk fire can be used after a grounded footstool to trap them (use 2nd jump to cancel launch from footstool). Learning how to set up some quick PKT2 stuff is nice as well. uair and utilt both can set up at various % pretty quick deaths for your foe if they don't watch it.
Oh and PK Jump and stuff is a thing. Pulling it out maybe 1x a match or more in a 3 stock game can be useful.
Off stage edgeguarding PKT1 can set up for bat, nair, uair, bair, regrabs, and more. Its more than just free damage. Very good for poking people who want to recover low. To get them to fly towards you on hit just snipe the direction they are facing.
Oh and cause I forgot it as well. Learn jab locks with Ness. Nair at mid %s can set up for a charged bat kill. Late Bair at high %s also sets up for a jab lock.
Dtilt when ledge guarding is good as well, oh and trips with it.