Setup a safe working environment lifting the Mustang with a jack and supporting it on jack stands. To make sure that the car does not slide off the jack stands, support it by placing supportive stands under the axel.
Carefully crawl under the vehicle and remove the rear Mustang tires and set them aside. Use a tire iron to remove the wheels. Make sure that you place the wheels outside the working space so that the wheels will not create hindrance while you work underneath the Mustang.
With a 3/8-inch ratchet and socket, first remove the bolts on the axle responsible for holding the upper control arms in place and then remove the bolts on the rear end which hold the shocks and sway bar in place.
With the jack, further lift the axel. Remove the bolts holding the pumper of the rear control arms in place. Slowly start to lower down the jack until you see a couple of springs popping out of the pumper. Pull these springs out and stop lowering the jack.
Remove the rear control arms by removing the bolts with a 3/8-inch ratchet and socket. Install the new rear control arms and reinstall the bolts that you just removed. Do not tighten the bolts completely. Instead, let the bolts remain a little loose.
To make the newly installed rear control arms functional, install the rubber spring pocket and slide the springs until they are at their correct location. You may have to lower the axel in order to accomplish this.
Reinstall the bolts at the front side of the rear control arms in order to hold them to the frame. Bolt in place the rear shock mount and sway bar and reinstall all the bolts that you had removed earlier.
Finally, reinstall the rear tires and remove the jack. You have successfully replaced the rear control arms on your Mustang.