Lay a sheet of paper out flat, and slide the piece of wood underneath it until it's at the center of the sheet. Decide on the dimensions of your floor cushion. The finished cushion will consist of a cylinder with a round top and bottom and straight sides.
Cut a piece of twine to the required length. If the round top of your cushion is to measure 3 feet across when finished, divide the measurement in half to give 18 inches. Add 2 inches for seam allowances and 3 inches for tying knots, then cut a length of twine of 23 inches.
Tie a slipknot in each end of the length of twine, and measure the distance in between, which should be about 20 inches.
Put a pencil through one slipknot, and the point of a thumbtack through the other. Push the thumbtack through the paper into the piece of wood. Use the pencil to draw a circle with a diameter of 20 inches. Cut the circle out.
Fold the fabric in half, widthwise, with right sides together. Pin the paper circle to it and cut around it to give you two fabric circles. Use a piece of twine to measure the circumference of the circle.
Cut a rectangle of fabric where the length is the circumference of the circle and the width is the required height of the cushion plus 4 inches. Pin the short sides together, right sides in, and machine sew along the seam. Pin the long side of the tube round the circumference of the top circle, with right sides together.
Use scissors to snip around the edges of the circle to open the edges of the fabric and make it lie more neatly against the straight edge of the rectangle. Make each cut 1 inch long.
Machine sew around the edges of the circle with a 2-inch seam. Pin the bottom circle in place and snip around the edges as before. Machine sew around the edges, leaving a gap of 4 inches. Turn the shape the right way around by sliding your hand into the gap, taking hold of the fabric and pulling it through. Run your hand round the seams to push them out fully.
Fill the cushion with polystyrene beads until it's plump and well-stuffed but not overflowing. Hand sew the gap closed using a slipstitch.