It screws into the same metal bracket that the ash tray screws into. You just remove the two screws from the ash tray, pull it off, then screw the cup holder back on with the same two screws.
The holders are fairly close together, so they work well for two medium fast food drink cups but only one large drink cup. But I usually just put a large in the square hole up front.
This is a really inexpensive, simple to install, and incredibly useful mod. I would highly recommend it to anyone looking to add cupholders.
Marlin - I think new from Isuzu the part was like $25-30.