Since you are new player, I recommend you to l-cancel with a light input on either of the shoulder buttons or z. That way it will not mess up with your techs.
Practice l-canceling by doing a lot of aerials from different positions. When you hit somebody, there's going to be hitlag frames which change the timing on which you hit the ground, so you must learn to both l-cancel hits and whiffs. Hitlag is the same for you regardless of if you hit a character, shield, shy guy etc. The stronger the aerial, the longer the hitlag is.
Practice l-canceling on visual cue, because then hitlag and different jump heights etc don't mess you up. When you practice, for short periods at a time concentrate only on getting the l-cancel. Learn also to act asap out of your characters l-canceled moves, because then you can take full advantage of the cancel and you will also quickly learn when you succeeded. I suggest at first try to jab out of the l-canceled aerial asap.
L-cancelling is largely just muscle memory once you get it down. You L cancel before you hit the ground, don't know the specific frames but you can look it up, it's somewhere. Best way to practice it is to start making an effort to L cancel during matches.
One should really try to avoid l-canceling on purely muscle memory, because it won't work when circumstances change, like hitlag, or different aerial timings. Don't worry, it will become automatic even though you don't do it on solely muscle memory.
