I found this tutorial at Michael Miller for a pillow case with flap. (scroll down until you see it on the right) I used the measurements indicated except I made 2 of the 12x21 and omitted the 8x21.
I installed an invisible zipper on the long side of the 2 12x21 pieces. I'm assuming here that you know how to install an invisible zip. Close the ends like you normally would.
One side attached.
Once that is done, you can stitch up your sides. Pin your case together matching your seam lines and all raw edges.
Remember to open your zipper!!
Stitch both sides, finish the edges and turn right side out. Press your case. Don't press out your crease you made earlier.
Be sure to poke those 2 corners back to the inside.
If I haven't made something clear, please feel free to ask questions. I don't know if this is the easiest method out there, but it worked for me. And trust me when I say I tried many approaches.