You've gotta be extremely trigger happy with your shield while against Sheik. She can camp with needles and rack up quite a bit of damage on Falcon if you're not near her at all times. Jumping around the needles isn't exactly smart since she can punish you upon landing with them as well.
Keep the pressure on her. Bair is a good tool for pressuring at lower percents so you can try to avoid getting trapped in some Ftilt/Dtilt/grab combo. Utilt is decent for pressuring, but Sheik has range and speed, like us, so you've gotta be really careful with what you do and avoid getting predictable. Sheik can get around most of our options without much effort.
That being said, Sheik is terrible in the air. As much as I love her air game, it's absolute garbage compared to most characters. Her Fair and Bair are amazing for pressuring, her Uair is decent for punishing characters above her, but her Dair is laughable. It's only good for mind gaming, imo.
That being said, just keep the pressure on her with Falcon. There's not really much Falcon can do against a character like her because she has a projectile; a fairly good damaging racking projectile. So don't give her the opportunity to put it to much use and just stay in her face. Like I said, be really defensive and use your shield as much as possible and you should be alright.
