Not much to this one. As long as you have room somewhere to squeeze another pot in the enclosure and it's in a spot that you don't mind drilling a hole for it you're golden. You just need to add the pot between the power jack and the circuit. I did one for a friend recently and I ended up putting the starve pot on the side of the enclosure so that it was out of the way.
Beavis Audio has a good, simple explanation of what you need to do.
http://www.beavisaudio.com/Projects/DBS/