I made this travel/toddler pillowcase for a kids' sewing class I'm teaching. The kids are sewing full size pillowcases, but I'm using this one as an example. The cuff isn't sewn on yet, but you get the idea.
For this size pillowcase cut two 12" x 15" rectangles and one 23 1/2" x 6" rectangle. Sew them up using a 1/4" seam allowance and presto - pillowcase.