Select a Site with Good Drainage, free of weeds or choking grasses. If any exist, remove by pulling, tilling, or smothering with newspaper or plastic.  Use of herbicides are discouraged and should only be used if absolutely necessary carefully following label instructions.

Loosen soil by tilling to a depth of 4", then rake smooth. Avoid fertilizing: this only encourages weed growth and will produce extensive leaf growth at the expense of blooms.

Control of Weeds
Weeds will choke out wildflowers unless removed.  Do this at and near the site you have chosen for wildflowers BEFORE you seed. After the wildflowers have germinated, it can be very difficult to distinguish flowers from weeds. For this reason, its best to wait until blooms appear, then simply pull up the unwanted weeds. This means waiting 8-10 weeks, so if you do recognize a weed in the meantime, obviously get rid of it.