I made Alex a sheet for his pack and play since that's what he's been sleeping in since we've moved. I followed the directions exactly but what I didn't take into account is that we placed a couple of pieces of foam on top of the bottom floor piece, which makes it a little thicker. This was my first attempt and it fits perfectly if I take the foam out. But I'll be making another that is bigger so it accommodates the foam.
This is the fabric I chose for the sheet, it coordinates perfectly with the colors and theme of the pack and play.
There are a few tutorials out there but here is the one I used: Prudent Baby