Hi, I personally use gorgeous Guineas shampoo as it won’t damage the oils. And is very natural with ingredients that are kind on the skin. I believe there are other products specifically for pigs that you can buy at the pet store. I personally wash my Guinea Pigs about twice a year. As even with good shampoos I don’t like to bathe regularly.
Please don’t be tempted to use baby shampoo as it is a common misconception that it is safe to use whereas it still damages the natural oils on Guineas skin as it is designed for humans not delicate piggie skin.
I bathe in a small wash bowl with a cloth in the bottom so they don’t slip. You could always just go with a ‘bum’ bath if it is just the bottom end that needs a tidy.
I find my long haired Guineas do need bathing a little more often but usually a bum bath will suffice during gaps between bathing.
When bathing be careful not to get water near there eyes or ears and ensure they are dry before returning to the cage as you don’t want them to become ill from still being too damp. I use a hair dryer on a low setting for my long haired pigs but a bath towel can also suffice for short hair pigs.
Hope this was helpful