Perform a soil test in late June or early July, at least two months prior to tilling the flower bed. Plan to till the bed in the late fall or early spring, when the ground is thawed but the plants are dead. Collect a soil sample and mail it to a testing laboratory or your local university cooperative extension, following their specific sampling instructions.
Analyze the results of the soil test to determine what, if any, soil amendments are needed to balance out the soil in the flower bed. Purchase amendments using the results as a guide. Check to see if the soil analysis includes recommended amendment rates, or consult with the staff at a garden supply store when purchasing the materials. Unless you're trying to grow flowers that require specifically acidic or alkaline soils, aim for a final soil pH of 6.5 for most flower beds.
Spread 2 or 3 inches of compost or another organic material evenly over the flower bed. Apply up to 6 inches to try and correct overly sandy or silty soil, but excessive organic materials can increase the chances of a grub infestation. Till the soil to a depth of 8 to 10 inches.
Apply any necessary soil amendments evenly over the flower bed. Also, cover the bed in 12-12-12 fertilizer at a rate of 2 lbs. of fertilizer per 100 square feet of flower bed.
Till the flower bed to a depth of 8 to 10 inches once the amendments and fertilizer are applied. Make several passes with the tiller to ensure all the various amendments and the soil mix thoroughly.
Water the soil to the same depth you tilled it once per week if you tilled the flower bed in the spring. Repeated deep watering helps to settle the soil and encourages the amendments to mix with the ground. Continue until a week or two prior to planting.