Been covered a few times

did mine a little while ago, this is what I did and seemed to be the general consensus on the forum...
Pull seatbelt all the way out and shut in the door, with as much of the belt outside as possible (avoid the door latch!)
I used a pressure washer, but if you haven't got one I'm sure a hose could be nearly as good. Hold onto the belt though to avoid the clasp hitting the bodywork!
Next, use an all purpose cleaner from tesco or similar, or I used vanish oxy action in a washing up bowl. Scrub with a nail brush and work the APC or similar in with your fingers.
Finally, rinse thoroughly, repeat cleaning as required, but once finished rinse as best you can.
I then left the belts shut in the door to dry off, so had waited til it was a sunny day to do this! Also helped the drying process along with a hair dryer
Hope this helps, Jon