Go to Kadano's thread and look at his up throw follow ups for Sheik.
Punishes in this matchup mainly involve getting as much damage and advantage as you can out of a single throw. This means you should practice the follow ups Kadano lists as well as follow ups to those. You'll start to build certain juggle strings that will lead into up air juggles for damage OR opportunities to carry Sheik offstage with F-air/N-air, after which you'll be setup for edge guards. The ideal punish on Sheik puts her offstage without her jump with the opportunity to edge guard. On stages with platforms your aerial combos can lead into tipper F-smash so look for that where possible.
If there were a bread-and-butter for Marth it would be his ability to patrol stages and cover landings with short hop aerials. This is something Marth can do in a lot of matchups to exploit the difficulty most characters can have with landing vs. Marth. The key here is to mainly use short hop, rising aerials to catch them trying to land. Full hop isn't recommended because if you miss, you're still airborne for a while before getting to swing again, creating an opening. If you want to mix up, mix in some double jumps after your short hop so you can fast fall quicker. You can also mix in some up tilts, which can kill Sheik at a decent range (especially if you hit them with the back swing.) Using these tactics on Sheik will help you build a punish gameplan rather than just looking for true combos all of the time.
