Okay so, I actually have a lot of experience with this match up, around this skill level. So I know what to do.
FIRST OFF
UPTHROWWWWWWWWW Uthrow->knee works around 70-80% to about ~110%. That should be your PRIMARY form of killing in this match up. I really like CF vs floatyish characters [Marth, Peach, Sheik] because it's really easy theory wise, it consists of getting damage up to that percent and doing the throw->knee to win. I don't know if you knew that, but you NEED to do that from now on. If you don't do that you will fail. You. Will. Lose.
2) You really have to control this match up. The best way to take control is grab. When you've grabbed, they literally can't do anything except DI once you've thrown, or try to hammer out. As far as grabs go, JC grab. JC grabs work by pressing y just before you press z. Grab->dthrow chains until about 30% [theory] or 13% [anecdotal]. Wheel of Fortune [dthrow->uair->dthrow] works about 40%-60% I think. They can get out of it if they decide to DI away. Also, uthrow->tech chase stomp on platforms absolutely wrecks people, but luckily it seems you've picked up on that. Don't forget that Stomp-> knee works just as well as every other character, it's just the opportunity is much less frequent.
3) Don't feel like you have to approach. This is a match up where neither person wants to approach, because if they mess up at all, that's death for either of you. In NTSC much more, because of sheik's Dthrow, but even so, a ftilt combo will mostly kill you. What you should do is dash dance until she throws needles, then go in there with something. Just make sure you don't Fastfall, because if you fastfall you'll get hit by them.
4) Learn when you can knee.
Here and
herehere you had a beautiful knee ready, but you used a uair instead. There are times when you can knee out of nowhere, like if they miss a ledgehop fair and you're just outside of range. Knees will wreck sheik, it's just hard to hit.
5) Edge guard her! Stay on the ledge until she explodes, and make sure you follow her teleport. That's the main way of removing a sheik from this earth if she's ever offscreen. If she teleports straight up, roll from the ledge. If onto the stage, ledgehop knee, if at the ledge, roll. Hope you didn't ledgehop, it gives her the ledge the second you're off it!
6) Random tips: make sure that when you ledgehop knee, you do it as soon as you grab the ledge, so you're still invincible. Don't use RB except in a combo, or as a tech chase mechanic. Even on shield, it's not safe. Don't approach with it. Also, make sure that you remember that Sheik's tech is like half the screen.
Don't try to reflect needles, just jump around them. Don't spend a lot of time in your shield, you'll die if you do. Spend most of your time DDing.
If I remember anything else I'll let you know.