we have 3 guinea pigs about 5 years old. They need their nails clipped but I'm very nervous about doing it. All their nails are black so i can't see the quick. I accidently clipped one too short a long time ago and now I'm spooked that i'll do it again...plus they are very squirmy and hard to hold onto even with help. Any suggestions and how can I tell the length to cut if i can't see the quick?