You only need to give them a bath every 4 - 6 months. Rodents are very meticulous and will clean themselves multiple times a day. However, you do need to have their nails clipped every 6-8 weeks. (We do it ourselves.)
When giving them a bath, we hold ours under the faucet to get them wet (with lukewarm water). We then use Bunny shampoo around their body and rinse them off. Do not put their heads under the water. Also, washing them too frequently can dry out their skin.
Hope this helps!