It is a myth that you can throw out some seed, water and have a wildflower garden. Although wildflowers will eventually naturalize and become very low maintenance, some preparation in the beginning will increase your chance of success. A wildflower is a species that has shown itself to be hardy and self-reproducing with little attention from people.
A sunny to partly shady site with good soil is best for a wildflower garden. Remove all of the existing vegetation. It can be removed by hand, sprayed with an herbicide or you use a method called solarization in which you mow the area short and cover with a plastic sheet for 6 to 8 weeks to kill existing weeds and seeds. Remove the dead vegetation before planting. Resist the urge to till the soil deeply because you will bring more weed seeds to the surface. If you must till go no deeper than 1-3 inches and then rake and level the soil.
Choose seeds that are native to your area or an adapted species. Starting with a mixture that is developed for your regional climate is a good idea. Over time the flowers best adapted to the site will thrive. You can also choose individual packets of seeds. The advantages to sowing individual varieties are that you can easily identify seedlings and track their success or failure because you know what you planted. It is important to include annual seeds for first year color as well as biennial and perennial seeds for subsequent seasons. Perennials and biennials may not bloom the first year. Packaged seeds will tell you how large an area they cover. A good rule of thumb is 4 oz. per 2500 sq. ft or 4 pounds per acre.
Planting in the fall gives some perennial seeds the conditions they need to break dormancy. You can also plant in the spring and summer as long as you have at least 8 to 10 weeks before a frost to give the flowers time to set seed. Some wildflower seeds are very small so mixing the seeds with sand makes even spreading less difficult. After broadcasting the seed, rake gently, water well and remove any new weeds by hand. Lightly mulching with peat or compost will keep the birds from eating your seeds and preserve moisture. You should see sprouts in 10 to 21 days. It may be necessary to reseed some annuals like corn poppies and cosmos in subsequent years because they do not reseed well.
After your wildflower garden is thriving the largest maintenance chore is to mow the garden at about 4 to 6 inches in late fall after the flowers have gone to seed. This will tidy up the site and ensure the seeds have been distributed for the next year. This also keeps any woody weeds from trying to take over. As your garden matures you can identify bare areas and spot transplant or reseed to achieve the look you want.