I made a couple of them for some camp rifles- what I did was...
Fold the blanket in half then set it, nice side out, on a large woorkbench. Put your rifle, barrel about 1" from the fold on top of it and adjust for the nicest pattern (if the blanket is striped or something). Mark a line about 1-1/2" around the rifle in chalk- leave about 8-10" at the butt. Add another 1/2 to 3/4" and cut out the blanket (use a sharp scissors or utility knife, just don't cut up the dining room table too much.
Turn it good-side in and sew along seams. I use a double-needle running stitch 1/2" in from the edge, and then a whip stitch to "roll" the edge along the long seam. At the muzzle end I add a bit of scrap leather for reinforcement and a little extra protection.
Turn it right-side out and add some ribbon or lace ties to secure the butt-end flap.
Hope this helps,
vic