Here is a simple pillowcase pattern. It's dimension are a little smaller but you can go with the ones on the Ytube.
Body of pillowcase is cut 27"with the fold on the bottom, then trim off the selvege on the top. Open the cut body and attache the cuff to the long side. Begin sewing case together at the cuff, reinforce your first stistches, continue down the side and then along the back and your case is done. I usually zig zag over the seams to keep pillowcase from fraying.
http://conkerrcancer.org/pillowdirections.html