Okay, for starters, you need to understand L-canceling has 3 different sweet spots that are to be hit when SHFFLing. There's the missed one (nothing is hit), the one where you hit your opponent (slightly delayed), and the one where you hit your opponents shield (slightly more delayed). The easiest way to L cancel is to perform your aerial attack, and after the animation begins, press down on the control stick to fast fall. When your character is about to hit the ground, you press L. It sounds trivial, but pressing down on the control stick helps significantly when L-canceling. Additionally, Project M has a very nice feature to it: when your character successfully L-cancels, you flash white for the cancelled frames. Basically, you can see if you're L-cancelling if you see your character flashing. My last bit of advice is to really focus on your character. If you focus on what buttons you need to hit, you'll be able to hit them properly in sequence. It's all about the timing.