What materials could I use to get a real smooth nymph body ?
The material must be easy to use when weaving using the simple ‘granny weave’ method. I have stranded silk floss, but its too difficult to weave.
Could you include a link to the material.
Qg.
Since you raised the issue of “smooth”, I guess Larva Lace or Larva Lace ‘midge’. Not so smooth you could use embroidery floss, rayon or similar. Rough you can use any wool or synthetic type yarn.
Deezel
Try using embroidery floss. It’s available in many many colors and holds together well for the overhand weave (as well as others). It’s fairly smooth but will still give you the ribbing effect that you want for most nymphs. Best of all, you can get it at any craft store or even Wally World.
TxEngr
The secret to smooooth bodies is having smoooooth fingers. Here is the link. 
http://flyanglersonline.com/about/flytyerstreatment/
I’ve used Larva Lace for years. You can see some of the patterns at: www.troutseeker.blogspot.com