Measure the height and width of the Parson's chair seat. Measure the height and width of the chair back. Measure the height of the chair from the floor to the top of the back.
Sketch the measurements of the chair onto a piece of paper to help you create a paper pattern. There are four pattern pieces: two pieces for the back to create a pillowcase, a chair seat piece and a skirt around the chair legs.
Add 1/2 inch around each pattern piece for seams. Pin the pattern to muslin fabric and cut it out to make a test slipcover.
Pin the right sides of two chair back pieces together on the sides and top. Sew a 1/2-inch seam around the fabric to create a pillowcase. Turn the slipcover so the right sides are out.
Pin the seat cover fabric to the open front edge of the back covering. Sew the seat cover onto the slipcover with a 1/2-inch seam.
Pin the top edge of the chair skirt around the slipcover by attaching it to the seat cover and rear edge of the back cover. The back cover opening will remain open so you can pull the slipcover over the chair. Sew the skirt to the slipcover with a 1/2-inch seam.
Pull the slipcover onto the chair to verify the fit. Adjust the seams to improve the slipcover fit, if needed.
Remove the slipcover from the chair. Hem the bottom edge by turning under 1/4 inch followed by another 1/4 inch around the bottom edge of the skirt. Use pins or iron the hem to hold it in place for sewing. Sew the hem close to the inside edge.
Adjust the paper pattern based on the test slipcover. Cut out the slipcover from a medium-weight fabric and repeat the steps to create a parson's chair slipcover.