If he approaches with a SHFFL or SH aerial, I like f-tilt, fair, and d-tilt for defense. Especially d-tilt. If he approaches from higher, I like bair, up tilt, and f-tilt as a defense. But fair also works.
I really like crouching in this matchup because it's difficult for Falcon to do tight movements efficiently because of how much ground he covers with everything he does. So by compacting your body when you're not moving, it makes you that much better defended from his typical "spacing aerials" like nair and whatever. He has to aim lower or predict you moving out of crouch, which makes his air vs your ground game a lot more simplified, which Sheik likes.
If Falcon doesn't DI away from your throw, you can combo most of your relevant ground moveset on him (basically any tilt, jab, and down smash). Knowing this can make for some good combos. Also, Falcon can ASDI out of your jab resets at 32% so don't do those after 32%. He can SDI out of them below that and it's not too difficult, but if you hit one you can fairly easily hit him with the double up smash hit, which leads to massive damage and often good combos (to perform, dash inside his body as he's standing and JC up smash; both hits connect; it's easiest when he DIs away but you should be up tilting on non-DI away most of the time anyway).
I'd talk about ranges but my opinion of significant ranges for Sheik is sort of under construction for this MU at this point so I'm not really sure what to say about it. So maybe Teczero or someone can field that one for me.