A guinea pig should only be bathed once a month usually at the most. I suggest every 4-6 weeks. Short haired pigs may need less washing while long haired pigs may need it every 4 weeks.
Always follow the instructions on the bottle! Do not use human shampoo on guinea pigs because human shampoo is designed for humans and is not suitable for use on animals. You can purchase standard small animal shampoo from most pet shops and suppliers.
A great place for guinea pig shampoo, a brand I personally use, is called Gorgeous Guineas. They have a wide range of products that are all safe for guinea pigs. They have fast shipping and great communication.
During medical treatment my vet has previously given me prescription shampoo complete with instructions. It is especially important to follow instructions if given shampoo by your vet and all medicated shampoos must be used under veterinary guidance.
What you need
* Your guinea pig. You can't give him a bath without him or her.
* A plastic washing up bowl or your kitchen sink.
* Towels (old ones!)
* Wear old clothes because you may end up getting soaked.
* Small animal shampoo or recommended product.
* Measuring container or jug.
Your guinea pig must be completely dry before he is allowed to continue his day. This is especially important if your guinea pig lives outside or in shed that has no heating.
Keep the setting low on the dryer, and always at a safe distance. These photos may look like they are close up, but you should always have it placed a good 10 inches or more away from your guinea pig. Many guinea pigs have ended up being badly burned..