If you've got the rifle, then make the pattern from that. Set the butt of the rifle down on a piece of paper and draw around it. Now add about 1/2" or so to the outside edge all around to allow for stitching. Next tape a few sheets of paper end-to-end and roll the stock (side) completely around on the paper, marking as you go. Add another 1/2" to that pattern all around to allow for stitching. Now transfer the patterns to leather and cut it out. Buffalo hide works great for buttstock covers.
I like to stitch them up inside out, and them turn them rightside out after I'm done, as it hides the stitching and give some extra cushion in the butt of the cover. I leave a slit down the side, with leather lacing, for easy on/off and adjusting the fit.