With a scrub brush.
Or, you can wrap the cloth parts and control box in duct tape, then scrub down the plastic with some cleaning stuff.
Moist towelettes should work too, but make sure to dry the pad of thoroughly afterwards.
Oh, another thing you can try it to pull of the dirt/gunk/whatev with tape. Box tape works pretty well for that.
_________________
I used to be active here lol