I would try it with muslin first because I havent done this but if I were to do it then this is how i would do it lol.
lay the pillow on the muslin use fabric pencil/chalk to draw around the pillow and then increase that drawing about 1/2-1 inch then cut two and sew them together leaving a end open to insert the pillow. giving yourself the extra 1/2 inch to an inch will make it sew you can put it on the pillow then determine how much and where to take it in for a good snug fit.
Sorry if this doesnt make sense, i typically use measurements now to sew and do it all by sight unless it is something super complicated.
I also found these link that may help
at 12:53 AM on Feb. 5, 2010