Let's get the trickiest part of this costume over with first: the hair. To capture the unruly nature of the troll doll's hair, I suggest buying a long, relatively plain, wig. I did some serious damage to mine, so there's no need to get anything high-end.
First, brush out any tangles in your wig. It's important to keep in mind that the longer your wig is, the bigger a challenge it will be to prop up and walk around with. I hung mine upside down, and gave it a bit of a trim. Be sure to keep some of these scraps to cover the netting later on.
Once your wig's the desired length, dunk it in some glue. Elmers is fine. The hair on your wig is plastic, so don't bother with proper hair products - glue worked better. After you've fully saturated it, brush through again with the comb as best you can. Then hang up overnight to dry.
After the hair's dry, use your comb and tease the hair by brushing down towards the "root." This will add a lot of volume and cause a bit of "troll dandruff," but that's easy enough to shake out.