I'm looking into the same for our 89 BII. They work by installing into the current spring pack. And they are very stiff and need to be just about pressed into place using C-Clamps. They will add more arc to your current spring pack and give you a couple of inches of suspension lift. The side effect is that you're ride in the back will be a bit stiffer--but it's a trade off that is acceptable to many people.
You can check the lift kits on other sites, such as Summit and just look over the instructions and that will give you the gist of how they work.
S-
(Let me know how you make out. I'm waiting until spring/early summer to do ours.)