For measuring your trapezoid bay window cushion, Focus on the front width first, then back width, then depth, and lastly the thickness of the cushion. First, decide on the area you want your cushion to cover. After that measure the widest part of your window seat, then coming to the narrowest part, and lastly the depth from front to back at the deepest point. At last, determine the desired thickness of your window cushion. We provide an easy-to-follow guide, or you can create a paper template of your seat shape and send it to us. We’ll take care of the rest.