I would take a piece of gift wrap tissue paper and tape your patter to the tissue paper. It usually comes in 20x30" sheets. Make sure the pattern piece is smooth as best as you can. If you tape a pencil to a ruler and a stiletto or another pencil 6 inches apart on the bottom and follow the curve. That should work. Newspaper would work but has ink. Tissue paper or freezer or even parchment. A brown paper bag may work also. Just extend your markings. Another way to make your tree skirt is to take your fabric and fold in quarter on the diagonal then round the bottom. many patterns for making a tree skirt. Just trim the way you want. Pretty much the same way you would make a round table cloth.