Definitely watch the videos. Your major problem is speed, you need to be pulling out moves a lot faster, definitely abuse your fair and dash attack, learn to space your dash attack better, try to hit Marth with the last spark, it's got more range than his tipper. You've got the right idea getting in close, but use your tilts(especially your ftilt and dtilt) If Marth is in the air, or just above you, start juggling him once or twice, and if you think he's going to use his counter, purposefully jump into him, and hit him with a bair once the counter is over, it's a good mindgame if you can predict the Marth.
Other than that, just be careful around Marth, don't run into his range unless you have an attack, and just...just freaking STOP with the Pk Thunder. Especially on Pokemon Stadium, seriously, only use PK Thunder if he's above you and you can't juggle him.
Practice your DjC more, against Marth, you need to be able to do to DJCFFLs pretty often. Other than that, spacing, spacing, spacing, abuse the bair, and learn to love your fair, the basics for a Ness.