Ok, you said you wanted a perfect circle around the crepemyrtle. The easiest way I have found is to take a length of string or light rope that is half the diameter of the circle you want. Tie a loose loop around the base of the
tree. Then with orange marking paint(the can will spray upside down) slowly walk around the
tree at the end of the rope/string while spraying the paint. When you connect the ends you have a perfect circle to start with.
If you use grass killer spray, make sure you protect your crepemyrtle with something like plastic or cardboard, so no spray gets on the
tree. Most if not all grass killer sprays kill everything they touch.
The least work will be putting down newspaper or cardboard and then put good topsoil on top of it. However, make sure you do not pile it up to much around the
tree, it may rot.
If you decide to remove the grass, there are tools specifically made for that, but I have found a good sharp shovel works well. I always take off at least an inch of dirt with the grass.
So, have you decided what kind of border yet?????
Hope some of this helps.