Fast falling can only be done as you reach the peak of your jump, so it's important to learn that timing and visual cue so you know when you can input it as early as possible. The L-cancel timing of a SHFFL is later than the fast fall as it needs to be right before you land. I recommend practicing the fast fall and L-cancel timing separately so you can learn each of them properly. Practice fast falling at the peak of your jump, then practice L-cancelling a SH without fast falling, then once you are comfortable with each of them on their own, combine them and due the full SHFFL. Don't get impatient and try to do everything at once; dividing things up may feel tedious, but you'll end up saving yourself time in the long run.