Using an inexpensive plastic skeleton, some paper mache and some expanding foam you too can have a "dead alien" this halloween.

Step 1: Ingredients and Tools

Ingredients:
Inexpensive plastic skeleton
Expanding spray insulation
Masking tape
Wire or coat hangers
Balloon, size of alien head. Have a couple spares just in case...
Paper Mache supplies - paper, paste, etc.
Blood colored paint (red, green, blue, purple, it's your alien) - if spray paint, use plastic friendly type
Gloss coat - spray or liquid
Gray paint - or any other acceptable alien skintone.
Black acrylic craft paint or india ink.
Optional - latex paint, any light color, or white glue.
Optional - lenses from dollar-store sunglasses
Optional - packing tape
Tools:
Slicing/dicing utility - knives, rotary tool, laser, what ever you feel comfortable with.
Rubber gloves
Pliers and/or wire cutters

Step 2: Adjust Your Skeleton

Start with a cheap plastic skeleton (known as a "Blucky") from your local store. I got mine a Wal~Mart for $15 Canadian.
In order to make the alien a little odd looking, exchange the upper arm and leg bones - this will make the legs shorter and the arms longer adding to the oddness of it.

Step 3: Adjust Hands and Feet

With the assistance of a knife or a Dremel, separate the fused-together feet into three "toes". You may attempt the same with the hands, or just use wire to create them like I did.
Wrap the toes/fingers with masking tape to form, ummm, toes and fingers... I also used packing tape to stiffen the arm and leg joints. Be generous.

Step 4: Torso Enhancements

Step 5: Create Intestines

Carve out several ribs and the stomach area. By arching the back and seperating them a little, it expands the torso length of the skeleton, making your alien "taller". I also reversed the pelvis of the skeleton and cut a section out of the new "front" to change the anatomy of the creature.
Put on your rubber gloves and make sure that you are wearing some old or protective clothing as we start to get messy...
Shake your can of expanding spray foam. If you are not familiar with this product, it is a can that contains a liquid version of a urathane product. When you press the nozzle, the foam comes out and down a tube where it proceeds to expand to many times it's original size and is used to stop the airflow around windows, doors, cracks, etc. The key to using it for halloween is the speed of application - if you keep the nozzle in one area or slowly move it, you end up with a blob or a filled area. If you move it quicker, you end up with intestine-looking blobs, and if you move it quickly you end up with smaller intestine-looking blobs. See where I'm going with this?
My alien has some intestines coming out of the torso that we opened up earlier. In the upper area fill using a slower moving motion, and then as you get towards the lower area start to build up with faster moving strokes. Stop occasionally for a few minutes to let the foam expand, and then add some more smaller intestines, maybe even hanging some over the edge...
I've also added some in the pelvis to increase the stability. It did help.

Step 6: Add a Head

The most visible difference between the typical gray alien and their human counterparts is an unusually (for us!) shaped head. I removed the skull to use for another project.
Find an appropriately sized balloon for the head of the alien and tape it to the neck of the skeleton. Or not, it's up to you. Round balloons will give you a chin at the mouthpiece.

Step 7: Paper Mache and Intestine Paint

I started doing some paper mache, and while it was drying I sprayed the interior of the torso with red spray paint. Don't worry if it's too bright, we'll add some depth to it later. Leave some of the ribs exposed.
If your prop might be exposed to weather, you may want to toughen up you paper mache by adding white glue or latex paint. Make sure that you coat the fibers completely so that when it dries the mist/fog/rain/snow won't be able to get through.
Build the face of the alien using finely-pulped paper, or some other substructure such as packing tape. If you have a cheap pair of sunglasses with the appropriate shape, use them for the eyes by building the mache over the edges, sealing them in place. You may also want to leave flaps of 'skin" around the open wounds, whether they are on the face, body or appendages.
Add several layers of paper mache building them up and letting them dry. This step took a few days as the weather was already cooler and my workshop wasn't heated. You may also have to do one side at a time, unless you can find a way to hang your alien and let him dry.

Step 8: Skin and Bones

Paint your alien's skin. I used a gray latex paint that I had bought off the mistint table. Cheap!
If your intestines appear too bright and mono-chromatic, apply a "wash". Dilute some black paint with water until it's quite thin. Then using a brush spread it randomly across the intestines - it will pool in the low places and leave the tops bright. You may want to do this a couple times letting it dry between. Instead of black, you may mix black and red, or brown and red, to get different shades, and use this technique multiple times to build up more "depth".
Coat the intestines with a gloss coat. You can use spray, or something like "ModPodge" as long as it's gloss. This will keep them all wet looking.

Step 9: Placement of Finished Prop

Place your alien on an autopsy table, at a crash site, in an "open grave" or wherever it will scare the most people. Mine is lying right next to the front door of our house, in the debris of a "UFO" so that kids couldn't see it until they were at the door.