A Short hop fast fall l-cancel is an advanced technique, it is essential to many characters' ability to combo on and off the ground. It originally was a glitch in SSB as shielding overrides lag frames from aerial attacks.
The steps are as follows:
1. Short hop: This is done by lightly tapping the jump buttons (X or Y). Fox and Sheik, however, have the fastest reaction time to short hop and thus require very fast fingers to do so, whereas Ganondorf/Bowser are the easiest to do with.
2. Attack: Attack immediately with either the control stick or A, or with the C-stick. Most pros highly recommend using the C-stick in most situations as it does not influence the character's DI. Which attack you use depends on whether it needs to be L-cancelled or not. Sheik is the exception to the rule, as her aerials except for Dair need not be L-cancelled.
3. Fast fall: At the peak of the short hop, jam down on the control stick to make the character drop faster.
4. L-cancel: If the attack is still going within 20 frames before the character hits the ground, this is where L-cancelling is a must. Otherwise, you will suffer lag frames upon hitting the ground.