Double leaf, as in two new leafs per pack?
The single leaf kit (per pack) from ProComp has decent instructions in it, but I don't see how you could put two new leaves per side in your spring pack without new new leaf alignment bars. The add a leaf goes below the normal spring pack and above the overload spings. It's not captive so it could conceivably rotate and end a tire, save for the U-bolts - since it's not very long, the U-bolt should be enough. If you're thinking about doubling up an add-a-leaf kit, that seems like a really bad idea.
If you are talking about a single leaf kit, if their instructions are so poor, did they include a longer center bolt pin? If not, don't start until you get one or you will be sad.
1. Jack up truck, rest on stands, take wheels off.
2. Disconnect shocks. This is easier if you jack up the axle a bit to unload them; on my wife's 98, the stock length shocks are at least 1" too short in back.
3. Get jack stands supporting the axle.
4. Unbolt front and rear of the spring pack (rear is shackle).
5. Unbolt the U-bolts and drop the pack.
6. Use a vice grips to hold the round top of the pin (concave side of springs), clamp with two c-clamps to keep from getting whacked by the spring tension, and unbolt the center pin.
7. Grease the ends of the add a leaf (I did the center too), then use the new longer center pin and recompress as much as you can.
The instructions I got said "loosely" but I cranked it as much as I could b/c the overload spring was wiggly. Still had a LOT of slack after bolting up with the U-bolts. I also greased the front & rear hanger eyes since the factory grease was pretty much resdiue.
Before bolting back on the u-bolts, I connected up the front hanger. It made it a lot easier to jack up the spring pack to mate with the axle. I used a cutoff grinder to trim the excess length of the center pins before putting the wheels back on.
Things to make it easier:
Air tools
2 floor jacks
2 sets of jackstands
A helper for putting the bolts back into the shackles, the spring has to be pulled down while simultaneously shoving the bolt through.
Regards,
Brian in CA