Do you have the Modified Mustangs Feb/March Edition? If not, I can give you a quick rundown.
1. Remove the trim piece from around your shifter. It's plastic and held in by clips, just pull. Disconnect your lighter accessory. The clips comes right out.
2. Remove the four bolts holding down your shifter boot. I believe they were 5/16" or so. Unscrew the shifter knob, remove shifter and boot.
3. Remove the four bolts holding down your shifter. I believe they were 1/2". You'll have to pry up the old shifter. I used a small pry bar because the old silicone is pretty strong. Take off your old shifter. Clean around shift housing with a flat screwdriver. Put on a small bead of fresh RTV blue sealant around housing.
4. Install new shifter unit with the old bolts. To adjust the stop pins provided, you need to use the fully threaded allen screws, one on the top of the shifter plate, and one on the bottom. Be sure to put nuts on both ends of the screws to make sure they don't move when driving. Put the car in first gear, adjust the screw until it hits the shifter, back out one full turn, then tighten the nuts. Next, put the car in second gear, tighten the bolt again until you hit the shifter, back out one turn and tighten. This will stop you from bending then forks when banging gears. Once that is done, you'll need to put together the shift handle. You use the S-piece first, then the threaded knob second. Attach with one allen screw and one fully threaded allen screw for each piece. Use the supplied nuts on the full threaded allen screw tighten all parts afterwards. Put the shifter cover back on, tighten the 5/16" bolts, then put the trim piece back on.