When it comes to designing a garden bed you need to decide between a straight clean line or an organic curve to your garden. More often than not, a curve will look nicer and for that it is easiest to use a garden hose to lay out the curve. This will allow you to experiment with different shapes and sizes easily and help be able to visualize the perimeter before finalizing your temporary line with spray paint. For straight lines, circles, arcs and more a more precise radius you'll want to use a stake and a string. For a circle around a tree you can use a string tied loosely to the tree and then to a spray paint can at the appropriate distance away from the tree. Then, walk around the tree with the string pulled tightly to paint the circle.