you need to bait the spot-dodges with your dash dancing and THEN grab. You should mostly be grabbing/d-tiliting out of neutral and and especially grabbing after confirmation situations (especially whiffed grabs/dash attacks). As a general rule of thumb don't go for raw ANYTHING in this matchup because sheik is looking to just punish the lag on your moves and you're trying to do the same. Stay grounded because it's easy for sheik to get under you, so don't fair on their shield and just utilize your superior movement on the ground to trick the shiek and punish hard off of grabs (keep them above you or throw them off the stage). As for needles, your wavedash out of shield game needs to be really solid because if it is you can easily block the needles and wavedash out with a downtilit. After hitting the downtilit, you can either grab out of it if they're close enough, or run away to create space and frustrate them. It's challenging to pull off, but shiek actually CANT out camp marth if your movement and wavedash out of shield are solid.
So yes my number 1 advice that made me understand this matchup after struggling for so long is try to make her do something first instead of you. Most shieks single handedly destroy marths off of frustrating them into approaching. Both characters are fairly limited in their approach so either one can punish the other hard for making a wrong move. Don't get frustrated, play patient and don't spam but punish hard when you get the opportunity. If you can snipe her jump and keep her above you its actually pretty sad how screwed she is.
Keep in mind waiting for them to do something first does not have to be playing gay/campy, you can be aggressive about how you do it and thats what makes the matchup interesting. By moving close but not actually doing a move, you're creating tension and alot of the times you can bait out a dash attack which you can dash-away grab or shield-grab. This is a matchup where either character can zero-death the other in a single punish (albeit its slightly easier for shiek), so just remind yourself that there's no rush and that one extra second of not approaching may just give you the edge. Watch PP play against mew2king's sheik with marth and you'll see, he plays this style better than anybody else and makes it look simple. I hope I helped I used to get destroyed by spot-dodging, grabbing sheiks.
Son of Slobodan
has given a lot of great info. While I am still new here, from my experience the most important thing is your mobility. Use it to force her to attack you, and then wave dash back and punish. The shiek I play often (who is also very new to the game) loves to out space me and then grab or even dash attack to punish me, and wave dash back seems to be a great solution to stop these approaches. Only when you get shiek to freeze up and respect your mobility can you come in with d-tilt. Hope this helps!
