Step 1: Gather Your Materials
Step 2: Mark the Shirt
A real seamstress would probably pin the pattern and trim off the excess material. But I decided not to totally ruin my t-shirts, I may need to wear them again. So instead of cutting out the pieces, I left the sleeves attached. This will leave them bunched up inside the seat cover.
Step 3: Stitch It Up
Once the seam is done, turn the shirt back to right side out. It should roughly resemble the shape of the chair back.