I understand they are washable BUT do NOT use detergent. Get some soap flakes. If you don't mind risking suds everywhere, you can use a machine. It helps to shake/knock the worst of the bits off it before you start. Use hair conditioner instead of fabric conditioner at the end. Hang it out to dry. You will have to brush/comb the fleece to make it go fluffy again and you do risk pulling some of the wool off it when you do this, so be gentle.
When I had one years ago I waited for a hot sunny day, put it in the bath with warm soapy water and gave it a good swirl around before rinsing with tepid water.
When the worst of the drips had disappeared down the plug hole I carried it down to garden and laid it out across the top of my opened-out Hills Hoist washing airer until it was dry.