This prop was actually very simple, just clothes stuffed with newspaper, then wrapped in layers of saran wrap. I made the head out of paper mache and put one of those clear plastic masks over it, but you could also substitute a wig form or a mask stuffed with newspaper. It looks best if the face is pale, to make it look like the victim was drained of blood.
Since mine was going to be hung vertically, I decided to put a clothes hanger in where the shoulders are, and attach that to the belt loops of the pants with wire. You just have to use your best judgement to figure out where the shoulders are in relation to the pants.
Stuff the pants with newspaper (don't worry, the saran wrap hides a lot of mistakes!), and attach the shoes to the bottom of the pants with safety pins.
Attach whatever you're using as a head to the hanger with tape. Put the shirt on over this, and stuff it and the arms with newspaper. Don't worry if it's a little thin; it is an emaciated victim, after all! If you secure the newspaper with masking tape in a rough cylinder for the torso, you can let the shirt hang down naturally, rather than pinning it in place. (That isn't that big a deal in a wrapped up dummy like this, but if you wanted, say, a hanging victim, it'd look much better that way.)
I folded the arms up to the chest and safety pinned them in place. Hands weren't necessary; they wouldn't have been visible anyway.
Finally, go nuts with the saran wrap! It looks better if it's a little bunched up; that makes it look more like strands of webbing. Tie a rope around the loop of the hanger, hang the dummy up, add small fake spiders here and there, and a little fake blood (put down newspaper if you don't want the floor stained)... Add some webs for effect, and you're done!