Step 1: Materials
These alone won't be enough though, so head to your hardware store and grab a couple cans of the cheapest black spray paint you can get, and one can of similarly cheap red. Snag a can of expanding foam as well (great stuff). A box of 2" nails to hold it all together and you're all set.
Step 2: Gory Juicy Guy
Paint the whole thing black, then before it dries add some red while the black is still wet. The two paints mixing give a charred bloody look, and you can add more red or more black to taste for what you like.
Once dry you can either nail this whole cardboard plank to a tree as a standee, cut it out, or place it wherever you like. Two cans of great stuff would do good for really filling out the shape, but it's plenty creepy with one can. Just don't go for a full sized adult with one can as it'll probably run out 3/4 through.
Credit for this idea comes from the Cockeyed.com "How Much is Inside" adventures.