Flower gardens have different soil requirements from lawns. Ground for use as a flower garden will need organic matter to lighten it and may need additions to correct pH and composition. Perennial garden soil must be prepared for permanent residents; soil for annual gardens can be re-invented each year. The best way to start is to have a soil test done at your local state university extension and get the agent's recommendations for your soil.
Stretch a black plastic tarp over the garden site and let it "cook" for a two to three weeks to kill all vegetation. General herbicides containing glyphosate kill most grass and weeds, but you won't be able to plant anything for two weeks or more after using them.
Cultivate the entire garden area to a depth of at least 6 inches and break the soil into pea-sized pieces. Pick out rocks and rake several times to remove weeds and dead plant roots. Level the soil evenly.
Add at least 2 inches of a mixture of well-rotted compost with an equal measure of clean manure, humus or peat moss, depending on the soil's texture. Add more organic matter to a clay-heavy soil. Work the additions into the soil well. You should have at least 8 inches of rich garden loam when finished.
Blend in a general garden fertilizer; about 2 lbs. of fertilizer (or equivalent organic source) for every 100 square feet of garden. Give fllowering plants more phosphorus than nitrogen (which is present in the organic addition anyway), so look for packages with an NPK number of 5-10-10 or 5-10-10.
Correct deficiencies identified by soil tests. Add trace elements of copper and iron only in quantities specified in test summaries. A soil test also will specify how much compost and sand are needed to lighten a clay-heavy soil or how much manure and silt are needed to boost a sandy soil's loam.