I can't cut my guinea pigs' nails?

I have two guinea pigs and always struggle to cut their nails. I have tried having one person cut, one hold; wrapping them in a towel; using a torch to see the bloodline better, but they seem to hate it and constantly wriggle away which means that sometimes they bleed - then we use a styptic stick. Is there any way to make it easier? We got our vet to do it the first time, but he was terrible and made them bleed even more, plus it was really expensive. Please help - one of my guinea pigs has started biting when you try to cut them, and I don't want her to be scared.