Thanks for writing. I don't recommend using an herbicide and then planting your flowers unless you plan to spray several time. One application of herbicide typically isn't enough to kill all the grass.
Renting or borrowing a sod cutter is going to be your best bet for getting rid of the grass so you can be sure it doesn't come back.
Alternatively, if you're patient, you can also cover the area with a 6-sheet-thick layer of newspaper, then cover that with a couple of inches of compost. Wait a couple of months (or all winter) --- it will kill the grass underneath and, as a benefit, enrich your soil at the same time.
---Justin, Senior Garden Editor, BHG.com