My camera's not up to snuff, but I'll try to describe it. Also consider looking up Poncho Liner Underquilts. I've done one of those, too.
My underquilt, I measured 48" side-to-side and cut straight down the side, leaving the sleeping bag as long as it came. I'm 6'4", so you can never have too much length. I then pushed the batting back a bit and sewed the bag closed where I made the cut.
I then tied a larkshead knot onto each corner to suspend the hammock from the hammock's suspension. It's not perfect, but it'll suffice until I come up with the money for a proper quilt.
The top quilt was easier. I cut off the hood of the mummy bag, sewed it up at the cut, and all done.