We first used the garden hose to layout the shape we wanted the bed to be - then we went back and marked the perimeter with flags. It is a diamond shape but with rounded points.
This is the finished bed. We planted a total of 6 double knock out roses (3 to the left and 3 to the right of the tree), 4 abelia kaleidoscopes (bright green leaves - 2 in front of the tree and 2 behind the tree), and 2 loropetalum rubies (dark purple leaves - 1 in front of the tree and 1 behind the tree).
Unfortunately, the roses went into some pretty severe transplant shock (wilted and dropped their flowers) when we planted them. They are finally starting to recover now, and we are hoping to have some more blooms in a few weeks. I'll post some more pictures as they bud and bloom again.
On a somewhat related note, does anyone know what this is? It is on a few of the leaves of the maple tree and a Google search of "red bullseye on maple tree leaves" didn't turn up much info.