I've done it a few times. It's not hard. Start by removing the screws that hold the top and bottom panels onto the chassis.
Walter recommended a spray cleaner called Hosa Bigbath. Place one end of the little 'straw' into the can's nozzle and the other into one of the slots on the pot casing. Shoot a small amount of the spray into the pot and rotate back and forth a few times. You should feel the pot begin to travel more smoothly and the spray breaks up the gunk. Repeat as necessary for each pot.
I let the amp sit for a bit to allow all of the cleaner to evaporate before reassembly. Make sure you do this job in a well-ventilated area, or use a mask if necessary as you don't want to breathe the fumes from the cleaner. Nasty stuff.