snappy, go up 2' and see if the rock is in good shape, if it is, lay a 1by on the floor, measure 2' off of that, snap ya a chalkline on what needs replaced, if not do the same at 4'. I'd take a utility knife and cut it off, you could use a saw, but be careful of electrical wires and plumbing. now check and see if its 5/8 in stead of 1/2, it should be for code. take a pencil and mark where your studs are, on the floor, so when ya put up the new rock ya know where to nail. cut your new rock in halve(2'),or leave it at 4'. factory side up, cut side down. nail, or screw( ya want it just below the surface, but ya don't want to tear the paper) tape(cheese cloth, not paper), mud, first coat just covers tape, with a 6" knife, second coat covers first coat and feathers out any imperfections with a 12" knife,split over the tape(24" total), third coat is a skim (finish) coat, water down your mud and make sure its clean, any particles in it and it will drag in your mud and leave lines. sand, cut the top and bottom edges where it feathers into the sheet rock(sander horizontal), sand the rest as flat as possible with out getting into your tape( if ya get into the tape, just go over the spots again with a light skim coat),(sander at a 45 angle). primer, paint(flat hides the most, high gloss shows the most), trim out, caulk, and set back and smile at the job ya did!. good luck, if ya got any q? pm me, little d.