You level and grade the surface, then pay some unscrupulous gypsy-types to pave it on the DL for cash. Spent some time in that line of work.
By the square foot is for suckers, do it by the cu ft of material or just as a flat rate on the job. Most pavers will rape you any way they can, i.e.
x square feet at 5.25 in of material uses less asphalt than a full 6, and won't be noticed by the poor fellow until his driveway cracks prematurely, which won't be for some time.
The prep work is really just making sure that the surface is level, firm, and properly graded. A lot of crews salt under the area to be paved to prevent plants from busting up through, and if there is any vegetation at all, douse it with herbicide before you kill it or dig it up to be sure it's dead all the way down to the roots.
Also look into having regrind laid down and compacted. Same process, but good, clean regrind runs 45-65 dollars per yard around here. Once it's compacted, it is a good hard surface. We did surprisingly many farms/businesses this way and had a lot of return business.
Remember, by the square foot is for suckers.