I found it shocking to learn that changing pad covers cost more than the actual changing pad for a changing table. As usual, I decided to tackle the project myself to save a little money. I had no idea how to do it, but it wasn't too hard to figure out a "good enough" way.
I simply sewed a hem around the edges of the fabric, then laid the fabric upside down on the changing pad and pinned just the corners, taking up the excess fabric at the corners (like you would when wrapping a present.) Then I sewed a straight line down where I had pinned to form a corner and repeated for all four corners. I turned it right side out and slipped it over the changing pad for a semi-fitted cover. The professional covers have elastic around the bottom (like a fitted sheet) but I find it looks exactly the same if I just tuck the ends under the pad. I was able to make three covers for less than the cost of buying one.