Take careful measurements of your chair from the top of the back to the front of the seat. Place the measuring tape directly on the rail and hold it firmly while you work your way down to get a correct length measurement.
Measure the width of the chair from one side of the frame to the other. Take a couple of other width measurement from different parts of the chair as well to make sure you are getting a correct measurement. Take one at the seat and also at the half way point if you have a longer lounge type sling chair.
Use the longest width measurement as your guide since you can trim excess fabric.
Purchase a new sling with these length and width measurements.
Remove the ends caps of the chair using a screwdriver to help you pry them loose if necessary.
Unbolt the side rails using a wrench. Put the bolts in a plastic sandwich bag to use later when adding the new sling.
Remove the old sling. Remove the plastic spine to use with the new sling.
Insert the plastic spine into the casing on both sides of the new sling. Start with one side first and slide the fabric and the sling into the rails. Repeat with the other side.
Replace all of the bolts and tighten them down to keep the sling in place. Replace the end caps.