There are a few reasons someone would want to install an add-a-leaf to there truck. One reason is for increased towing capacity. Another more common reason is to lift the rear of the truck. Many people will put a leveling kit on the front of there trucks and an AAL in the rear. Its an easy and cheap way to lift your truck.
The directions below will be similar for other makes and models as well, so feel free to use as comparison even if you don’t have an f150.
The first thing you will need to do, is jack the rear of your f150 off the ground. Place jack stands under the frame, and lower truck onto the stands.
Then place the jack under either the left or the right side of the rear axle (whichever side you prefer to start on). Remove the u-bolts from that side, and support with the jack. Lower the axle slightly to give you some room to work with the leaf pack. Watch carefully to make sure you don’t stress out the brake lines or anything.
You will then need to remove the pins that hold the leafs together. On the 04-08 f150 there is two of them. If you have an impact gun you may be able to remove the nuts with that. If not you will need to hold the bottom of the pin with a pair of vise grips, and then loosen the nuts.
You will also find two clamps one towards the front of the leaf and one towards the back. Pry them both open. Then slip the add-a-leaf in, and insert new longer pins. Tighten new pins down, and bend the clamps back around the leafs.
Now you are ready to u-bolt it back to the axle. Use the jack to align everything up, and then tighten the u-bolts.
Repeat on the other side of the truck. Then put the wheels back on, and jack truck off the jack stands and lower to the ground.