Clay soil is the soil that is compromised of very fine minerals particles and doesn't have much organic material. They're dense and resistant to water movement, does not drain well. Most of its mineral particles are very fine clays.
Soil that consists over 50% of clay particles is referred as "heavy clay".
To improve clay soil, you'll need to add 6 to 8 inches of organic matter to the entire bed, like shredded leaves, grass clippings or compost. And mix them into 6 to 12 inches of soil.
Over time, the garden bed will settle down as the organic material breaks down. The soil structure will continue to improve as microorganisms in the soil develop.
The flower can be plantted immediatelly after improvement work. Plan to add more organic matter in the form of compost once or twice a year.