Lay your cutting mat on a sturdy table. Unfold your fabric to a single layer and lay it on your mat. Place one of your curved templates on the fabric near the edge of the fabric. Curved templates are available at most quilting supply shops and are either complete circles or are curved parts to a standard pattern set. Make sure the fabric and the template are on the mat.
Retract the guard of the 18-mm cutter and place it along the edge of the template. Cut around the template with the cutter. This method requires close attention so that the cutter stays snug to the template as you cut. You are also limited by the size of the templates that are available.
Another method for cutting circles uses a compass style cutter. Adjust the blade placement for the size circle you wish to make. Make sure to add 1/4-inch for seam allowance unless you are cutting circles to be sewn with raw edges. Firmly hold the pivot point of the compass with your left hand and guide the cutter with the right hand. Reverse hand positions if your are left handed. This method takes practice to work well.