I already have a round circle, how can I add a skirt (fire pit cover)?

The skirt, if it is to be not ruffled, is the circumference of the circle. The circumference of the circle is determined by this formula: pi * radius squared. The radius is one half way across the circle. Take that distance, multiply it by itself, then multiply it by 3.14. This will give you the length around the circle. The width of the strip is the height of the fire pit. So, if your fire pit is 4 feet across, the radius is 2 feet. Multiply 2 x 2, then multiply this by 3.14. This will give you the length. If your fire pit is 1 foot high, this is the width of the strip you need. Make sure you allow for a seam allowance of about 1/2 inch wherever you will be sewing.