Use a piece of cardboard folded in half. Edge them into a corner of their cage by slowly moving the cardboard towards them and blocking their exit from the corner. When they are right in the corner use both hands and cup them together over the guinea pig's body and scoop them up onto your chest and keep your hands over them, holding them in a gentle grip in case they struggle and fall.
Stroke them while you are hlding them - they rarely bite unless really hurt or frightened. They may squeal a bit and even pee on you (they do this even if happy and relaxed!).
Slowly dispense with the cardboard and you will eventually be able to scoop them up just using your hands.