I had an old GI Blanket and never used it. I stole this idea from several other on the Internet and did it my self. I used a sweater that fit well as my pattern.
Step 1: Cutting It Out
I wanted it to hang down farther on the back so as I lay on the blanket I adjusted the fold to my liking. I put a sweater on the blanket as my pattern. I ran a piece of chalk a few inches away from the sweater. I just cut along the chalked line with a pair of scissors.
I then marked it a hands width from the arm pit corner so as to add a diamond shaped gusset for the armpit.
Step 3: Sew It Up
Remember to pin it before you pick it up off the floor. Then measure where you want it sewn on the side. I wanted to be able to get to my pants pockets. I also added some black nylon to the cut exposed edges to prevent raveling. If I had had access to an serger this would have been a better choice. I had no serger so 4 dollars of nylon tape did the trick. I used hem tape and an iron to stick the tape down to sew it up.
Step 4: Aint It Fashoinable?
It is comfortable and VERY warm but ugly as homemade sin. My coworkers were either making wizard jokes or sound like the soundtrack from a western. Oh well I like it.