You can easily sew new covers for round seat cushions using upholstery or home decor fabric, or even a simple but sturdy fabric such as calico or denim. To make the cushion covers the right size and shape, you can measure the cushion to figure the diameter, or simply place the cushion on a large piece of paper and draw around it to make a pattern. This technique works best for round cushions that are no more than two inches thick such as cushions for stools or dining chairs.
Place the cushion on a large piece of paper. Draw a circle that's larger than the cushion all the way around, so that the circles will be large enough to cover the cushion with 1/4-inch seam allowance. Cut around the circle to make a pattern.
Make a second circle pattern the same size. Cut this circle pattern in half.
Cut two strips of paper, each 2 1/2 inches wide and as long as the straight edge of the half-circle patterns. Tape the strips to the straight edges of the half-circle patterns, overlapping them 1/4 inch.
Pin the whole circle pattern and the two half circle patterns to the fabric. Cut one whole circle and two half circles from fabric.
Hem the straight edges of each half circle using a 1/2-inch double fold. To do this, fold each edge over 1/2 inch, then fold it over 1/2 inch again. Press the folds flat and stitch across the straight edge of the half circle.
Place the whole circle and the half circles right sides together, lining them up along the outer edges. The straight edges of the half circles should overlap. Pin the pieces together.
Sew the edges of the round cushion cover together, using 1/4-inch seam allowance.
Turn the cover right side out and insert the cushion through the opening in the bottom. Leave the cushion cover open if you want to be able to remove it to change it or wash it, or hand-stitch the opening closed.
You can add piping or lace to the round cushion cover by cutting a piece of trim long enough to wrap all the way around the edges of the cushion. Sew the ends of the trim together, then insert the trim between the whole and half circles when you pin the edges together.
Use caution when working with scissors, pins, irons, sewing machines and needles.