Be reaaaally familiar with the character. Develop the muscle memory for Ness' little quirks. (Downsmashing to damage opponents behind you, being able to tail whip with PKT, and knowing how to PK Fire in midair and actually land it.)
If you can't beat level 9 CPUs with Ness, start there.
Practice PKT2 at various angles. Decide "I'm going to PKT2 at THAT spot" then be able to do it.
Learn how to use PKT for every scenario. Learn how to juggle and learn how to defend with it. (For example, lots of characters like Bowser/Yoshi, Link/Tlink, ZSS/Sheik/Sonic will try to use a rapid fall move on you. Be able to defend against it with the tail of PKT and curl the head around to hit them. The tail interrupts all moves and will save your life.)
Break habits other characters may have given you. This means never down/uptilting unless the very specific scenario calls for it. In this Smash, Ness should be using his specials and aerials nonstop, with yo-yos to deal with people too close, and the bat for reflection and KOs. The bat is SITUATIONAL. As good as it is, don't rely on it. Ness should be getting most of his KOs from aerials, back throw, and PKT2.
Last, but most importantly, practice your mechanics. These will help you no matter what character you play, but for characters like Ness that have their share of weaknesses, they're going to be extra important. Being able to save yourself with good mechanics will be very important. With good DI, powershielding, spot dodging,
teching, and fastfall timing, Ness is significantly better. (Also much hardier and surprisingly hard to KO.)