Mostly wiring the ABS sensors in and plumbing the ABS pump and computer into the system. Might require some brake line running.
I don't know the specifics of it, but i've been meaning to convert my Fox to ABS as well. I made sure i installed axles with ABS rings and hubs with the ABS rings on it so i'd be good to go.
I do remember a write-up, but forget who wrote it. I beleive a member on corral.net by the name of stangplus2birds did ABS onto his Mustang and made a write-up. I can't find it right now