Give us a bit more info. Dear Fairfax - How inventive. First you should decide whether you want a raised bed or one that is flush to the ground. In either case, you want to make sure your lines are straight using string and stakes. But before you plant, figure out a floral color scheme and ask your local nursery for ideas about what flowers look good together, whether you want all one color or high contrast.
Bagged soil is preferable to most of the dirt around here.
American Plant Food on River Road in BEthesda specializes in organic fertilizer. We wanted something that looked nice but still casual. Cost was a major factor as was quality. Thought I'd share our good find. Dear sofas - Thanks for sharing, and happy snuggling.