While guinea pigs can be bathed they are very clean animals and generally do not need it. However, there may come a time when you need to give your cavy a bath, for example if they get lice or wind up getting into something they should not. Veterinarian's Best makes a waterless shampoo for small animals that needs no rinsing which is good for guinea pigs who can't handle a water bath.

Start with warm water and a gentle shampoo for small animals or baby shampoo make sure you support the guinea pig in the water. Avoid getting water and shampoo on their faces or in their ears. Get a good lather going and massage your guinea pig checking for any lumps or spots on their bodies that shouldn't be there.

Also make sure you rinse the shampoo off thoroughly, if you don't get it all it all out it can irritate their skin. Use a towel to dry your guinea pig and keep him or her warm until they dry completely.

Here's a video of a guinea pig that gets a bath once a month, and seems to enjoy it. This lady holds her guinea pig directly under the faucet and doesn't use a pan, that is entirely up to you.