Wave dashing is, in short, used for two things:
1.) Main movement option to get around the stage (Luigi's main movement option)
2.) A quick repositioning which should help get out of danger or run in for a punish (Jiggs can wavedash to get a rest in from a roll read).
In a game, you don't have the time to readjust the analog stick to get a further distance in your slide, usually, in training, you will go twice as fast as you do in a match against a person. A tip to go further is before you jump then wavedash, simply dash forward and jump before the white smoke appears at your feet. When doing this, you will slide during the jump squat frames than the normal distance you have gone. However, you will learn that wavedashing is most useful to get around platforms, the ledge and jump a quick slide to reposition your character, NOT YOUR ONLY MOVEMENT OPTION. With Jiggs for example, I found out that I prefer to simply walk or run or jump around because I can do things immediately out of them. Wavedashing, though you're moving is still punishable and you can't do crap until you are done sliding.
TL;DR: Dash forward first then jump immediately to get more distance. During your squat frames, you'll slide forward a bit. Don't rely on wavedashing as your only method of moving around, unless it is a character who relies on it. Figure out the opportune time to wavedash, such as combos that can be started/extended, things like tech chases and jab resets and approach options with your character.