Mine always try to escape and end up soaking me and the floor if i try to do it in the sink or washing up bowls, so i cover the base of the bath in warm water so that the water just touches their tummies then turn of the tap, and gently scoop some water in my hand and soak their bodies (not the face or ears, just the body) when they are wet i rub some johnsons baby shampoo into them and lather it up, fill up a bucket with warm water and using a jug i rinse their bodies thouroughy with the water from the bucket, i usually let the plug out as i am doing this as the water level gets higher, when they are completely rinsed, i gently but firmly run my hand down their body to get any excess water off and then get them out and put them on a towel and towel dry them as much as i can then use a hair dryer to make sure they are totally dry!
They are so fluffy and clean when its all done! You can get special guinea pig shampoo from Gorgeous guineas, but so long as you dont use any ordinary shampoo on them, you'll be ok.
Though be wanred, they do like to shake when they are wet so you do get sprayed, i usually put a non slip mat on the base of the bath so they dont slip over when shaking.
Hope this helps.