I hope you don't get discouraged if it takes more than 20 minutes to make your pillowcase, because I have made a few of those, and perhaps could put it together in 20 minutes if everything else was cut out and in order, machine threaded and set up, and test stitching completed. Start to finish, just relax and enjoy the process.
Another way to think of cutting width of fabric is to think of it as across the grain. You are no dummy. You are smart enough to ask for clarification.