1. Cut a 41-inch square from both the red corduroy and the green velveteen fabrics for the skirt fronts.
2. From both the red and the green cotton fabrics, cut a 41-inch square for the skirt backs. To find the center, fold the red skirt back into quarters, creating a smaller square. Lightly press the folds to serve as placement guides; unfold and smooth out the fabric, wrong side up, on a flat surface. For the tree trunk opening, draw a 4- inch-diameter circle on the skirt back, centering the circle on the fabric where the creases intersect. For the center back opening, use a yardstick to draw a line from the center to the outer edge along one of the creases. For the green skirt back, fold the fabric into quarters diagonally, creating a triangle. Continue with the red back.
3. With right sides facing, pin the red skirt front and back together along the edges and drawn lines. Using a 1/2-inch seam allowance, sew the outer edges of the red front and back together, leaving a 6-inch opening on one edge for turning; continue sewing 1/4 inch from the circle and on both sides of the center back line. Carefully cut on the drawn line and trim the seam allowance of the circle to 1/4 inch. Trim the seams, and clip the curves. Turn the skirt right side out; sew the opening closed. Press.
4. Pin the green plaid ribbon along the outer edges of the red tree skirt, mitering the ribbon at the corners and turning under the raw ends at the center back. Sew the ribbon to the skirt along the long edges.
5. Repeat for the green skirt front and back and 1-1/2-inch-wide red plaid ribbon.
6. With right sides up, layer the red skirt on top of the green skirt, aligning the edges of the tree trunk openings and the left and right edges of the center backs. The green skirt will extend beyond the red skirt at the center back. Topstitch the skirts together 1/4 inch from the center back and trunk opening edges.
7. For the ties, cut the 7/8-inch-wide red plaid ribbon into six 18-inch lengths. Sew the ribbons in pairs along the left and right center back edges, positioning them at the top, bottom, and center of the red skirt.
8. Cover the three button forms with green velveteen fabric, following the manufacturer's instructions. Sewing through all fabric layers, center the buttons on the front and side edges of the red skirt, 3-1/2 inches in from the plaid ribbon. Sew a red tassel to each of the four corners of the red skirt and a green tassel to each of the four corners of the green skirt.