This is what somebody told me, who is actually quite an expert on such things:
Put it in the bath tub with washing-up liquid. Leave it there for a whlie, and if it's very dirty, leave it over night. Let out the water, shower it off, and leave it there to dry.
Apparently, that's everything. I asked if it would need polishing etc., and apparently it doesn't. Good thing, too; would be very time consuming.