I took my serger (a type of sewing machine that cuts the material as it sews) & took off a few inches of sleeping bag at a time, starting with removing the zipper only, then the hood, & slowly enough material that it fits nicely, but not too small. The sleeping bag zipper didn't go all the way down, so I left a foot pocket. My sleeping bag is an OLD, (about 20 years I think, maybe more) sinthetic, so I do need a new overquilt, but it works for now. It is now about 60% as big as it started, I took off just enough that my Patomic underquilt now fits inside the stuff sack the sleeping bag came in.
Total sew time, about 10 minutes.