Depending on the particular boots, they should be very easy to care for. You may be able to put them in a washing machine on a gentle cycle (on low heat); this will take care of the worst of the dirt, and they can be air-dried for the best results. If the instructions say not to put them on a machine wash, I'd say the best way is to just scrub them with warm, soapy water and leave them to dry in the sun: this is the best way to remove stains.