Fill a sink or tub with very shallow tepid water. They can not stand water above the neck. Most will get very feisty! They'll try to jump out, so I think a tub vs a sink is better. Use a mild shampoo made for pets or a baby shampoo and just wash them from the neck down. Avoid the eyes, ears, nose and mouth! They are very sensitive to that and also sensitive to temperature change so always make sure the water is not too cold or hot. When done, rinse with a shower head (on low) or a cup to rinse.
RIGHT when they are done, get on towel to dry em off as best as you can and another towel to snuggle in while still being damp. They hate the cold. They need. That reassurance that they will be warm again and ok. So baths are very tricky for a few times until they know what the outcome will be.