It just takes a lot of practice and patience dude.
But I'll give you some general knowledge.
If I was you, I would learn how to waveshine without hitting anyone first. I believe the shine acts slightly different when it hits someone and when it doesn't. I'll say this now, when waveshining make sure your actual wavedashes are long. You can do this by aiming the joystick as horizontal as possible when air dodging.
You can also try and think of the shine and wavedash as separate if you wish. But when you finally get the hand of it, you are going to start thinking of it as one move.
When you do master waveshining without hitting someone, I'd recommend practicing waveshining Link first because Link goes a very short distance when shined. In fact, you don't even need perfect wavedashes to catch Link, although you still need to make your wavedashes as long as possible. Once you can easily waveshine Link, then do it on Captain Falcon (he goes slightly farther) then finally on Marth.
Good luck!!! Don't quit.