Getting a neat, well balanced profile can be tricky with Czech-style nymphs. Here's a short sequence from the River Fly Box showing how I approach tying in the shell back for both Czech Nymphs and Shrimp (Gammaris sp.) patterns.
1) With the ballast in place (flat self-adhesive lead) I tie in the shell back, protruding over the eye.:
*Note* The shell back needs to be tied in so the side you want showing uppermost on the finished fly is facing downwards when tying it in.
2) Dub the fly from the eye to the bend. The thread should be hanging behind the dubbing at the bend end of the hook:
3) Pull the shell back over the fly and secure at the bend with two tight wraps:
4) In open wraps, bring the thread towards the eye, pulling down to give the segmented effect in the shell back. I'm using Moser Powersilk 10/0 here -which is very strong thread):
5) When you finish the ribbing, build a small head behind the eye, whip finish and varnish. Trim the excess shell back from the base:
I hope this helps. Full tutorial can be found here: Czech Nymphs / Gammarus