Step 1: Materials
rope (not string. String is too weak.)
para-cord (stuffs amazing!)
plant roots ( gonna have to be pretty long)
If you know how to make string from long grass or plants, that works too! I think there is an 'ibles on that in the survival skills contest :)
Step 2: Rope
Para-cord works best but i doubt you can find any out in the wild. Unless you pack some just in case :) Braid the para-cord so its thicker. That way it will hold the barbs better,
Rope!! NOT STRING. String will work, but is too weak!! If you have alot of string, make a rope by braiding.
Plant roots work, but long ones are hard to find. If you find skinny roots, braid them like rope to make a thicker,stronger rope.
Vines are like para-cord only more rigid. If the vine is DRY, make it wet when you finish the barbed rope so it's less likely to crack and split. This also makes it easier to work with because it bends better.