Part of the beauty of Sheik is that she doesn't have one optimal playstyle. That being said, I feel that her defensive play is a bit over-hyped, as she really can't take a hit. Play how you feel comfortable, and if it works, it works. Personally, I play more aggressively, but I'm not afraid to sit back when I can and play a zoning game.
As far as practice is concerned, just practice. I've used this analogy before, and I feel that it really fits here: If you're learning to play the piano, and you want to sit down and play
this, you'll start by playing those first four notes, and then you'll play them again, and then again.... For individual techniques, you want to do them as many times as you can well. It's dull, but it's effective.
I honestly don't use full hop to Nair, simply because I don't feel like it. Because you have to release the needles as soon as you jump, unless you're comfortable with Melee Fox's SHDL, I recommend using one of the top buttons to jump. Personally, I use Z so that my short/full hops can be more predictable. None of that analog mess.
C-stick should definitely be set to Attack over Smash. Don't use Sheik's smashes unless you reeeeally know that you can; they're all laggy, and she can't afford the damage. Smash stick also screws up aerial mobility (so I've heard; I've not even bothered with it myself), which is key.
I set Y to grab so that dash grabs are that much simpler, but it's not too important. All a matter of comfort, really.
There is a thread on staling. I don't know that there's a consensus yet, so maybe keep an eye on that one.
Vanish vs. Uair is tricky. In order to set up a potentially awesome kill move, you need to condition them to air dodge your Dthrows, or else they'll escape the Vanish, and you'll suffer your lag. If they're not airdodging, you're actually probably better off not using Dthrow, as a stale Uair is less effective, and she doesn't have many options out of it relative to the rest of her kit.
Reading a roll is actually one of the few times you can feel good using a smash attack. If you time it right, and/or your opponent has a lousy roll, your recovery frames shouldn't be an issue.
Bouncing Fish off-stage kills uncharacteristically early for Sheik, but yes, it's risky. Always remember that you can execute the second kick with a button press if necessary.
Battlefield is a contentious stage. Some people love it for the new combo opportunities that open up with the platforms; some people hate the platforms that can interrupt her. Honestly, I feel that she does well on all the legal stages, except maybe Delfino or Halberd. Some people like Lylat, though I personally hate it. I'm not the right person to ask for that.
maybe I didn't explain it very well
