http://www.therangerstation.com/tech_library/1998_BodyLift.shtml
1) Remove anything prevent access to all the bolts - bedliner, interior carpet, etc.
2) Disconnect the battery as well as any cables that may be stretched during the lifting process.
3) Loosen all of the bolts.
4) On one side of the truck, remove the bolts and then jack up the body enough to slide in the new spacers. Install new bolts and begin threading them down but do not tighten.
5) Repeat previous step on the other side of the truck.
6) Tighten bolts and test drive, then replace any of the items you moved for bolt access.
That's my really short/condensed version of the process off the top of my head. Might have missed a few key details.