With my gp, I reach for him from behind (so he won't automatically bite my fingers) and then lift him up high enough so I can support his legs with my other hand. I'm pretty lucky, though. Hammy is very sweet and I've never had a problem getting him to come to me and sit still when I pick him up. A few people here have guinea pigs that automatically run when they are reached for (instinct kicking in, prolly).
I use my right hand to put under the front of my piggie and the left to scoop up his bum as I lift him. Hope that made sense. It's not nice to pick them up with just one hand - they're a whole lot of small critter and it's not comfy to squish them around like that.