I made some a couple of years ago. I just kinda made them up as I went. I measured the mattress, length and width, and then added about 5 inches on each side. I wrapped the fabric I cut around the actual mattress, flipped it over, and trimmed, then pinned up the excess on the bottom at each of the four corners.
ETA: Forgot to say after pinning, I sewed up the bottom pinned seams, one for each corner. (duh!)
Then I sewed in 1/4 inch elastic all around the perimeter.
I did it this way because I wanted to be sure I had a nice, tight fit, and it worked great! The elastic around the perimeter, with a generous amount of sheet on the underside, kept it snug.
Sorry it's not more specific, but it's really not that hard. You should give it a try.
