At low percents, try uthrow, rising uair, falling uair, re-grab. THen you can uthrow again to a full hopped FFed nair, and improvise from there.
From 0 to 20-30ish, is a good percentage to uthrow sh nair. You can get him before he hits the ground if you're fast enough. Plus, you can re-grab and do it again if you do practice enough.
The saving grace for Falcon though, is that he has a really easy way of killing Marth.
Uthrow Knee at like 60-90. Its insane how easy this connects. They either eat it, or Double jump THEN eat it. Try it on a cpu or something, and you'll get what I'm sayin. Hope I helped man.