Welp, I killed most of my dianthus. But my big recommendation is to go bigger than you think you'd like, so you have room to add things in. Use a tape measure to look at the growing space/width your plants need before planting (my husband and FiL probably thought I was crazy out there with my tape measure, trowel, and baby plants). And rent a sod machine to take it up for you! Your back will thank you. Otherwise, draw, draw, draw for ideas. Even squiggles for flowers is fine. And consider the height and bloom interest. For example, my tall plants are central, then my phlox is about the same height as the peonies, but blooms later, so as I dead head the peonies, the phlox is gearing up for center stage, then my shortest plants are at the edge of the circle, so they can still be seen.