Using any chemical like Roundup would not be beneficial because you have to wait for the grass to die and at least 7 days before you plant anything. You can rototill it up and then take all the clumps of grass out. I did it to a big part of my vegetable garden this summer and it takes some time and sweat. If you are growing in full sun, you have an array of flowers to chose from - it all depends on what you are looking to create. I love delphinium, sedum, sage, phlox and many different types of annuals. If you're going to get this done by Thanksgiving, then I would start tomorrow.