It goes without saying that flower beds can be any shape or size you like, but sometimes getting just the right shape is easier said than done. If you want a square or a rectangle, nothing could be simpler than drawing straight lines, and free form beds are just that "free form," so anything goes. So how do you make a circular bed without ending up with an "egg"? Do not despair it is actually quite simple.
Decide where you want the circular bed and roughly measure the space to determine whether there will be obstructions such as trees, immovable objects like hydrants, electric poles or buildings that might interfere with your design. Adjust the position accordingly.
Drive a long spike (large nail, sharpened dowel or small metal pipe will do) into the ground where you would like the center of your circle to be. This will be the pivot point for making your circle.
Measure a piece of string a foot or two longer than half the width---the radius of a circle---you want your finished flowerbed to be. Cut it and tie one end loosely enough that it can swivel freely around the spike about 6 inches above the ground.
Stretch the string out and re-measure it from the pivot point to the exact length of the radius you have decided. Tie a small knot at that point (or mark with a permanent marker) so you will not have to measure again.
Hold the string taut and level (parallel to the ground) with one hand and walk slowly around the perimeter using the pivot point to guide you in a circular path. Use a can of spray paint made for marking lawns (they have special spray nozzles that work upside down) to paint a circle as you walk, using the knot or mark on your string as a guide for distance from center.
Remove the spike and string. Use a sod cutter or sharp shovel to remove sod from the interior of the circle (save for bare patches in your lawn) using the painted line as a guide.
Fork or turn over the new bed to loosen soil. Rake smooth and plant. If the bed gets full sun, you may want to put taller plants in center and step them down in size as you move out to the edge, or place tall plants in a crescent toward the north, with shorter plants filling the space between the "arms". A spiral design works well too.
Edge the bed with rocks, bricks or other garden edging for a formal look, or leave without edging and add bark mulch to the bed for a more natural appearance.