You Will Need:

1x Single Size Fitted Bed Sheet (May vary depending on the chair dimensions.)
Thread
Needle or Preferably a Sewing Machine
2x Sets of Bum Bag Clips
Min. 7.5 meters of Elastic Strap
Min 2.5 meters of Synthetic Strap
Any spare fabric

Step 1: Marking Stitch Paths

Cover the bed sheet on a roughly 100x200 cm bed. Estimate a midline that halves the bed sheet. Then depending on the height, width and depth of you seating structure decide on where would the joint between backrest and seat would be and draw estimate outlines for your chairs backrest. You should end up with a rough plan that is shown.

Step 2: Outline Stitching

After you mark your outlines for you backrest, you can remove the bed sheet out of the bed. Now you have a top view plan with excess borders that go around the bed and wrap it. Fold the borders underneath the plan side of the bed sheet. Take a needle and thread to your hand and stitch borders and the top together over the outlines you have decided.

Step 3: Cross Patterning

Once you are finished with the outline stitching, you can now cut elastic straps of 60 cm. This measure may be acceptable only to the chair used in this project. You can measure your length by stretching out an elastic strap that goes around either the adjustment knob or the column, to its full length. Assuming that you have done with cutting your straps, you should be stitching them through the imaginary line that goes through the joint in between the seat and the backrest. Looping them to each other prevents straps dangling around.

Step 4: Elastic Strap

Most time consuming part of all the steps is this one. You first should have a long piece of fabric that you can turn into a tube by stitching one side. Without stretching anything you should stitch the tubes to the imaginary line that goes through the joint in between the seat and the backrest and over the cross-patterned elastic straps. The length for me is the width of 3 chairs roughly 150 cm without stretch. Then you will be passing shorter elastic strap through and stitch them at the ends. This will pull the bed sheet enough that when covered the elastic strap will reach its maximum length.

Step 5: Backstrap

In this step you will be preparing looped straps. For this I used the measure of three chairs next to each other again. This time I cut them into two. One 65 cm and one 80 cm adding up to approx 150 cm. Mark how long you want your loops to be and decide on how many loops you want on synthetic strap. According to that divide the elastic strap into segments and mark intersections with synthetic strap. Stitch them together and add Bum Bag Clips at the end of each strap for easy tightening.