What I do, is I practice in training mode, until I have the tech skill/ combo/ trick pretty easy to use. Then I practice it on CPU's, until it's second nature
Though I'm sure people would disagree with my method of doing this, I generally try to only practice against bots when trying to learn the skill all at once because I feel pressure while trying to use the technique properly. Bots cannot suffice for regular players, but they can be a significant help in learning things like low-jump attacks out of shield because if you're actually playing one of the few bots that can provide significant shield pressure, you learn how to do it while trying to get used to that kind of pressure.
But, then again this is all considering if you don't have anyone to practice with for weeks on end... But, generally training is just better with friends (Hopefully friends that are any good at Smash), and local players.
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On the subject of training... Does anyone have a suggestion how to learn to wavedash out of shield? I know the inputs and the like, but I've been having a really tough time actually nailing the technique down in this particular situation.