Measure the chair with the flexible tape measure. Follow curved edges precisely with the tape measure to get accurate measurements. Record your measurements on a sheet of paper. Draw a sketch of the chair to help you remember what measurements go with what sections of the chair.
Make pattern pieces out of cotton muslin to test your slipcover measurements. Draw the pattern pieces on the cotton using the measurements you took, adding 1 inch to each measurement to create a 1/2-inch seam allowance on all sides. Cut out the pattern pieces.
Place the pattern pieces on the chair and pin them to one another along the 1/2-inch seam allowance. Check to be sure that you have a snug but not overly tight fit. Make any adjustments necessary and mark your changes directly onto the fabric. Keep making adjustments until you are completely satisfied with the pin-fit pattern.
Transfer the modified cotton pattern onto the upholstery fabric with a pencil. Cut out the fabric. Sew the fabric pieces with their right sides together using a 1/2-inch seam allowance. Press the seams.
Turn the slipcover right-side-out to test its fit on the chair. When you are happy with its fit, hem the bottom of the slipcover to the length you desire.