This is dead easy if your pillow has a cover.
Uncovered pillow or pillow form:
In this instructable, I'm covering a 16x26 inch pillow, so I'd consider my measurements to be 17x27.
We'll be cutting out three pieces all together, one for the front and two for the back. Many other tutorials just have you cut one long piece, but the I believe the extra seams make for a better fitting pillow. :)