The best method I can suggest is to get the smallest set of nail clippers you can find. The ones for kittens or petite dogs work best. I also find the clippers that close like a crab's claw seem to do much less damage and cause less stress than the ones where you insert their nail in the round part and then depress the lever to have the blade cut the nail. As for exactly what length to cut the nails, this link is great:
Guinea Lynx :: Nail Clipping