Cut apart the cassette, extract all the plastic spacers and the two sprockets you need, and put them on. You might need part of a singlespeed kit to get it to fit properly.
You need to put the sprockets so they're two shifts apart, if you see what I mean, maybe more, cause the chain is quite thick and it won't fit if you keep them as close as they come on the cassette.
Then you hook up your derallieur, wind up the locking screws so it can't go off either edge, and it will work.