Fold the big towel in half. If you're using two towels, place them on top of each other with the right sides facing out.
Pin the open edges together, leaving a small opening to insert the filling later (about 3 inches should be fine).
Using a sewing machine or by hand, sew along the pinned edges, leaving the gap open. I used a 3/8" seam allowance but you may need to use a different allowance, depending on the thickness of the towels (1/4" for thinner towels or 1/2" for thicker towels)
Fill the bed through the gap with your chosen filling material, until it reaches your desired level of fluffiness. For my larger, heavier cat I used more filling than for the other two.
You can hand-sew or use a ladder stitch to close the opening. Or use your sewing machine. Push the filling aside in the gap area, and pin the gap.
Sew the gap closed with your chosen seam allowance then fluff up the pillow to distribute some of the filling to the gap area.
You can also add additional stitching around the edge for added durability and decoration. Or you can cover the entire seam with bias binding. I wanted this project to take me no more than 10 minutes and I skipped this option