Measure the length of your chair. Measure from the top of one sling rail to the bottom.
Measure the width of your chair. Measure the chair width at three different spots where the bolts and spreader bars are attached. Measure from center of the sling rail to center of the opposite sling rail. Average these three measurements and round up to the nearest 1/8th of an inch. This is your width measurement.
Determine the amount of fabric you will need based on this measurement.
Take the length measurement from your chair.
Add 3 inches to 4 inches to this measurement.
Double the overall measurement so that you will have enough spline for each side of the sling.
Contact a sling chair repair company and order a sling spline. It typically is sold by the foot.
Add 1 inch to your length and width measurements.
Mark your measurements on the fabric.
Cut your fabric.
At both ends of the newly cut piece, fold the width of the fabric over 1/4 inch, twice, making a rolled hem.
Topstitch the rolled hem and iron it flat.
Take a spline and lay it on the lengthwise edge of the fabric, leaving 1 inch to 2 inches of spline hanging over either end. Fold the fabric 1/4 inch over the spline.
Using the zipper foot on the sewing machine, stitch down the length of the fabric, encasing the spline inside the hem. Alternatively, you can pin the spline at either end of the fabric, remove the spline, stitch down the length of the fabric so that the pocket hem is the width of the spline, and then thread the spline through the pocket hem you have just made.
Reassemble your sling chair according to manufacturer's directions.