A calm second person!
I get v stressed cutting my girl's nails and they are so wriggly (and a bit bitey). My OH wears his gardening gloves and I pop the pig on his lap. He holds her so she's facing out away from him and her front paws are resting on his hands which are around her middle, gently - supporting rather than restricting. Her back legs are resting on his lap. I then snick the back feet claws (which goes pretty well). For front feet I get a baby sweet corn (or other fav food item) and place this between my fingers and her teeth! I try not to hold her front feet and use the clippers to push back on the gloves a bit to enable the claw to slip into the cutting space. (I use the clippers designed for small animals rather than human ones)
I'm not sure I'm explained that very well.
I've tried lots of ways and been bitten a number of times, had at least one melt down per session and this was defiantly the easiest! My other Guinea pigs were never this tricky!