The hotter and drier the site, the better, so start with south and east-facing slopes or faces. The cool, north facing slopes are good for ferns and woodland wildflowers. The easiest way to prepare an existing lawn is to rent a sod cutter and use it to remove the top three inches of sod. Do not turn the soil over, as this will introduce more weeds than leaving the soil untilled. Plant nursery plants for sites under 1,000 square feet or so, and seed areas larger than that.
Spring is best for planting and late spring is best for seeding.
Transplants can also be planted in early fall. Use three to four inches of clean straw as a mulch to keep weeds down. Planting plugs is better than seeding in that you can label the transplants as you put them in the ground to later distinguish them from the weeds. A second way to convert lawn to prairie is to mow down to the stubble, then spray Roundup. For your information, the active ingredient in Roundup is glyphosphate, which acts to interrupt the process of photosynthesis.