Measure from the point where the mattress touches the box spring to the top of the mattress. Add 3 inches (7. 6 cm) to that number. A 12-inch (30. 5-cm) deep mattress will have a drop length of 15 inches (31 cm).
Multiply the drop length by 2. A drop length of 15 inches (31 cm) gives you 30 inches (76. 2 cm). Add the sum to the width of the mattress. A 54-inch (137-cm) wide mattress results in 84 inches (213. 4 cm).
Multiply the drop length by 2. A drop length of 15 inches (31 cm) gives you 30 inches (76. 2 cm). Add the sum to the mattress length. If your mattress is 75 inches (109. 5 cm) long, your total is 105 inches (266. 7 cm).
Place the fabric facing down on a flat surface. Use a tape measure and colored pencil to mark the dimensions of the comforter. Use scissors to cut out the fabric. Repeat the process with the backing.
Place the cut fabric facing up over the batting on a flat surface. Cut the batting 3 to 4 inches (7. 6 to 10. 2 cm) wider than the fabric.
Place the backing face down on top of the fabric. Align the edges of the backing with the outer edges of the bias tape and fabric. Use straight pins to pin the backing onto the bias tape, fabric and batting.
Use a sewing machine to sew along the edges of the backing, fabric, bias tape and batting on 3 sides. Sew along the edges of the fourth side until you are 12 inches (30. 5 cm) from the end. Do not sew the last 12 inches (30. 5 cm) of the comforter.
Bend the fabric over the batting. Bend the backing into the comforter. Align the edges of the fabric and backing with the sewn edges on the comforter. The bias tape should be in between the fabric and backing. Use straight pins to hold the materials together. Use a sewing machine to sew the hole together. Sew along the edges.
Cut a length of embroidery floss as long as desired. Thread the floss through a quilting needle. Insert the needle through the top of the comforter. Bring the needle back through the top of the comforter. Remove the floss from the needle. Tie the floss in a double knot. Repeat the process as many times as desired throughout the comforter.
