Add the bair to your finisher list, short hopped and shffled bairs can come out really fast, especially useful for approaching projectile users since its priority trumps almost everything(except for Sheik, Fox, and Falco).
Also, try to focus a good amount of time practicing your Recovery, you need to be able to sweetspot somewhere near 80% of the time, from any angle, to be decent at recovering with Ness.
In addition, learn his bat's range. Really learn where the tipper is, and learn to use it. Knock people just coming up on a recovery, it might be even better than Marth's for that purpose.
Use Ness's dair, it's wonderful. It has its own built in mindgame with the lag, high priority, a big long lasting hitbox. I personally use it a lot when I'm trying to fake an enemy out, use a dair way above their heads and then come down with a bair or fair.
Other than that, using Ness is a combination of spacing, timing, mindgames, and an overall cohesion of movement. Ness moves from one attack to another in a swift flow of jumps, double jumps, shffls, and wavedashes. Don't allow your enemy to interrupt your flow, and he can become a powerful force.
For your specifics, his approach is difficult, definitely master the DjCffls until you can do them lightning quick, they're your best bet for approach. Learn the spacing on his dash attack, the popup from the last spark is good for combing into attacks, and it has crazy long range. Fake the enemy out, and wavesmash into them, Ness has awesome wavesmash. Jump around, and trick the enemy into coming into a position where you can fair or bair them, and then keep pounding them, if you're fast enough you can carry them across the stage with nothing but fairs. Use your dair to fake people out, and even use it to attack, try to come in so they think you're just bluffing, and then actually hit them with it. His uair is also great, it has a hitbox on the side of his head, not just the top, so you can use it as an approach move with a DjC.
Edgeguarding too is tough. You basically have 4 choices, none of them spectacular.
A: Bat. Use it to knock enemies off trying to recover, doesn't work on many characters sweetspots. A good trick to use is, if you know the person excels at sweetspotting their recovery, PK Thunder them just outside their jump range, they'll be a little more confused than usual, and a little less likely to sweetspot their attack, though it's not 100%
B: PK Thunder. Tailwhipping is extremely difficult, but against a Fox or Falco, it can mean instant demise. At the very least, it's something they're forced to watch out for. Stand so you're just over the edge, and if your enemy looks like they'll still recover, veer the PK Thunder back to yourself and hit yourself with it. Worst case scenario, you'll be out of range of the enemy's attack. Best case scenario, you hit yourself just as the enemy is recovering, and they get hit by the mighty PK Thunder 2.
C: Dair. Not a good choice in my opinion. Too many recoveries outprioritize it, the lag is difficult to measure, and the meteor spike doesn't kill often enough against skilled players. The biggest problem you might have with this attack is if you time it wrong, and end up too low on the screen to recover with a PK Thunder. Or at a bad angle. Or you just **** up your recovery. Ness should be off the platform as little as possible.
D: Physically going after the enemy with an aerial. I highly reccomend this one in most cases. An fair can usually outpriorities most enemy aerials, and while it doesn't kill, it can keep your enemy off the ledge until you get into a position to bat or bair. The bair....well that's just obvious. Hello killing move. And against Ganondorf and Captain Falcon, the nair is pure hardcore ****(use it just above their recovery, so you don't get grabbed).
You can also try some of Simna's more...complicated PK Thunder techniques. An old favorite of mine is the PK Thunder False Edgeguard. Get an enemy off the ledge, but stay close to the ledge, as close as you dare. Send a PK Thunder after them then. Use the PK Thunder in a way that they're forced to recover over you(figure 8s work, as well as a simple curve) but make sure that you lead the PK Thunder back to yourself near the end. Just before the enemy lands on the other side of you, smack yourself with a PK Thunder. Also, say your goodbyes to your enemy.
Hope that was helpful.
