You need all the strips that hold on the ground effects. I do not know if the kit includes instructions on the exact locations for installing, so you will need to source that from somewhere.
http://www.latemodelrestoration.com...3-Mustang-10-Piece-Ground-Effects-Bracket-Kit
You have the front and rear bumpers. You need the fender extensions for the front bumper, as well as the 2 piece side skirts. The ground effects simply slot into the metal strips, then are secured with some screws along the inner fenders and bottom of the vehicle.
You also need GT door moldings. The LX door moldings do not work because the "indention" is thinner and will not match the GT bumpers/GFX.
http://www.latemodelrestoration.com/item/LRS-20939B/87-93-Mustang-Gt-Cobra-Lh-Door-Body-Side-Molding
http://www.latemodelrestoration.com/item/LRS-20938B/87-93-Mustang-Gt-Cobra-Rh-Door-Body-Side-Molding
I cannot personally comment on the quality of reproduction side moldings, but I have heard mixed reviews, especially on the supplied two sided tape.
After that, a GT wing (and brake light), the Fog light support and brackets, and a GT headlight switch to work the foglights. If your LX is a factory 5.0 (not four cylinder) car then it will have the fog light wiring. Incidentally, if you are changing out the headlight switch it is a good time to do the Fog light fix so that you don't melt your new switch.
I think that is it.