Spills on felt can usually be easily sponged off. Wool has the wonderful ability to ‘wear clean’, in other words, it will hold a lot of dirt before it is noticeable, so frequent washing is unnecessary and environmentally undesirable. Also, depending on the vigour with which you wash the felt, a certain amount of shrinkage will happen, which on the one hand strengthens it, and on the other, makes it smaller. So care must be taken to be gentle.
Hand washing of felt can actually improve its quality and strengthen it.
Wash in hand hot water using any good bar or powder soap, rinse, squeeze in a towel, pull into shape and leave to dry.