Answer: First you will need to remove the existing grass including the roots, either by digging it out by hand, or by using an herbicide such as glyphosate (be sure to read and carefully follow all of the label directions including waiting periods before removing the foliage and before planting), or possibly by using sod cutter machine (these are available to rent from equipment suppliers.)
Once you have removed the grass, loosen the soil down about a foot or so and work in ample quantities of organic matter such as compost, well rotted leaves, or well aged stable manure/bedding. Then level the area and rake the surface smooth.
You should also test the soil and add lime and/or fertilizer based on the test results. Your local county extension should be able to help you with testing the soil and interpreting the results based on which flowers you want to grow.
Enjoy your new flower garden!
Q&A Library Searching Tips