I had two faded pillows that I made a few years ago for our front porch. I used outdoor fabric; however, they faded over time. I decided to make spring/summer slipcovers for them. (I am eventually going to make slipcovers for every season. So far, I just have Christmas slipcovers and these floral slipcovers.)
The first step was to measure the existing pillows. I flattened them the best I could to get an accurate measurement. I determined that the slipcovers needed to be 16.5 inches and I wanted a 1/2 inch seam, so this equated to 17.5 x 17.5 inches total.
Once I determined the size, I needed to square my fabric, so I would have two perfect squares. I used a triangular ruler and a yardstick.