Start with a skull!
If you're a reasonable person, you'll stop there.
If not, why not make a whole skeleton out of pumpkins?

Step 1: Pumpkin Shopping

Finding the right pumpkins can be quite a challenge. You'll have better luck with a rural grower with a variety of shapes and sizes... the soulless clones that make it to your supermarket are just too perfect.

You'll need five separate pumpkins:
1. The skull: Shaped like a pear, and as big as your head
2. Torso / ribcage: Tall, barrel-shaped, or large pear, and torso-sized
3. Pelvis: spherical (a soulless clone will work for this one).
4. Limbs: As tall as your femur. The taller gourds tend to have thick skin and are stronger than pumpkins with poorer posture.
5. Feet & Hands: spherical (clone OK). You may be able to use parts from the other pumpkins for these...

It's an odd experience to hunt for pumpkins with this twisted agenda; you're not looking for perfection, you're trying to see the potential for body parts. But the nice thing is, these are usually pumpkins that nobody wants, because normal people don't make skeletons out of them.
Of course, you're looking at some serious total weight by the time you find all of your pumpkins, so be prepared to pay for well over fifty pounds.

Step 2: Mark 'em Up

Draw guidelines with permanent marker, keeping in mind the eventual carving. If you don't like what you've drawn, you can erase with alcohol if you really want to. But you'll be removing all or most of the orange skin, so no pen marks will make it to your final creation.

Find some skeleton illustrations for reference, but don't go nuts shooting for accuracy; aim for the overall impression, not each bone. For instance, if you do 5 or 6 ribs, they'll read as a "ribcage" to the viewer, and you've accomplished your goal.

My diagram should give you a VERY general idea of where the bones are hiding.

Notice: your skull, pelvis & torso are all basically hollow objects full of guts... and so is a pumpkin.

Your pen marks on the skull, torso & pelvis should be relatively symmetrical.

"Arms & Legs"
A pumpkin this shape can yield more than one of each bone, so try to keep it as intact as possible in case you have to make additional bones (they can break, your dog might run away with one). Drawing (& cutting) along the pumpkin's natural vertical grooves can help here.

"Hands & Feet"
You can get the hands from anywhere on the surface, depending on what your fleshless friend is doing - use the top where the pumpkin curves into the stem for "bent" fingers (my guitar player's left hand)

"Pelvis"
One of the trickier bits... there's not much mass to a real pelvic bone, so you want to take some anatomical adjustments. A bit bigger means it will "sit" better, even as it crumbles into soggy fuzz.

"Torso"
Leave an oversized chunk of spine for the lower back; you may need to do some engineering later to get it to fit together here. The "scrap" comes in handy for additional sections of spine, so don't compost any large pieces until the carving is done!

"Skull"
You'll remove the... er... brains... through a hole you make located right where the spine would attach, so draw accordingly.

Step 3: Clean & Carve!

Remove the pumpkin's entrails; save the seeds if you're hungry.

Note: Keep your pumpkin intact until you're ready to carve it, to minimize future festering. Keep body parts in your refrigerator until your whole skeleton is complete (Surreal moments occur when you open the fridge for a midnight snack and find yourself face to face with a human skull).

Tools: No rules here. I use a paring knife for everything. I do have a great scooper (a pointed shovel with serrated edges and a lovely orange handle: see photo) that speeds up the evisceration, but apart from that, it's all about the knife.

I'm afraid that you're kind of on your own for this step: when it comes to achieving the actual shapes; you've got to just go for it. But here are a few tips:

The pumpkin is strongest at it's outer surface, i.e. the orange part. As you carve, you're removing that part, exposing the creepy, yellowish bonelike surface. Keep in mind that the farther "in" you go, the softer the material becomes. On the photo of the moldy skull, I left a bit of orange here and there, but on the full skeleton, you should whittle it all away.

UPDATE: Ignore that last sentence about "whittling it all away." For the 2007 model, I left WAY more orange, with the following results:
1. Lower stress: big strength increase resulted in more cooperative bones that were less likely to break during the engineering phase (step 4).
2. More pumpkinicity: it was easier to tell that this was made from pumpkins; folks "got the joke" from passing vehicles, not just up close during trick-or-treating.

Another UPDATE:Check it out: Pumpkin Skull Instructable

Be sure to do the skull first, because as soon as it's done, your creation has personality.

This is essentially a relief sculpture, and you're obviously limited by the thickness of the pumpkin. Keep as much intact as possible, for instance, you may not need to carve out actual gaps between every rib; you need them to touch to maintain their structure. So do only one or two gaps, in strategic places (like between the top two and bottom two ribs).

Hands and feet can exploit the natural curve of the gourd. Do a lot of "cobblestone-looking" bones there (see photos), with "tendons" running over them leading to the fingers. Ew, gross.

The long bones in the arms & legs need as much of the strength of the skin as they can get, so don't get too thin or carve too deeply there, or you'll end up with large, useless noodles of pumpkin. A tall, "arm-and-leg-bone" pumpkin often has a very thick skin (sometimes requiring the use of power tools. See Tom Nardone's Extremepumpkins.com for poetic pontification pertaining to pumpkins and power tools!).

Shoot for nice, rounded surfaces. The "point" that shows up between two "whittle" marks can be smoothed perfectly with the blunt edge of a knife or the back of your fingernail, but that tends to bruise your pumpkin and shorten the shelf-life.

Depending on what your plans are for "step 4," you may need to keep an eye on the future and adjust your work for engineering's sake. For instance, the heavy torso piece might be resting a lot of its weight against the base of the spine, so you might want to leave more material on that section of the spine than you would like to. And the top of the head looks better when it's nice and rounded off, but because you're using a pear-shaped pumpkin, you might be stuck with a flattened head; if you round it too much, you get really thin walls that won't hold up the weight of the skull. And, of course, if your skeleton will be laying down or doing something else less demanding than playing guitar, you may not have to worry at all about structural adjustments like these.

Step 4: Wire It Up

Display your skeleton however you see fit. Ideally, you need to put it all together without relying on the pumpkin to support itself, because decay makes your bones gooey.

The engineering can get pretty ridiculous if you let it, which I did for this guitar-playing skeleton:

I planted a wooden dowel into a barstool, added a horizontal dowel to handle the weight of the torso, drilled holes through the appropriate parts (spine) and "threaded" the parts onto the dowel. I made a few extra vertebrae for between the skull and torso sections, and for where the tailbone meets the spine.

The skull sits on a ~3" wheel-shaped piece of wood (wrapped in plastic so it wouldn't stick) which was screwed into the top of the dowel.

Arms & legs were wired together with thin welding rod and wire hanger. A horizontal rod with loops at the ends provided a "clavicle" from which the arms could hang.
Another option: for 2009's creation I used several ruler-like bits of wood with holes drilled in the ends, zip-tied together. Provided a platform for the leg and arm bits, affixed with hooks made from hangar wire. Not as "pure" as previous years, but held up much better to transportation (it spent time at my kids' school, and at two other locations, not counting my front porch!).

Note: For best results, shish-kebob the long bones with wire, then attach to the adjacent bones. Eventually you end up with a wire armature to support all of your pumpkin bones.

I wired the legs to the stool and affixed the guitar with wire hanger. Some black felt inside the skull added some contrast and creepiness (just begging for red LEDs - maybe this year?).

As I mentioned in step 3, it can be a really good idea to leave some skin. I was able to wire the more generously skinned fellow up with far fewer collapses and such, and he survived transportation on the roof of my car (in a plywood coffin, of course) far better than his predecessors.

The "Laptop" pose was more symmetrical than the guitar pose, which also simplified the wiring. The only caveat was weight: I left far more material on the pumpkin, so it was a few notches heavier and tended to want to fall forward, so I wired him to the downspout.
The book that the 2009 skeleton is reading is quite heavy and sitting on little bits of plywood. I shifted his torso back to balance out his legs a bit, after which he seemed quite comfortable.

Step 5: Other Ideas...

The guitar player skeleton appeared in 2005. In 2006 I made another skeleton, placed it in a plywood coffin, propped on my lawn: less engineering, but also less action.

I believe the next skeleton will be climbing out of the coffin...
UPDATE 2007: My belief turned out to be unfounded: the next skeleton was, in fact, using a laptop computer.
UPDATE 2008: No Skeleton. Gourdzilla, the Dino-Pumpkin instead.
UPDATE 2009: The Skeleton is back. This year he's reading "Upgrading and Repairing PCs" by Scott Mueller.
UPDATE 2010: Skeleton pumpkin carving a pumpkin. Only tangentially gross.

Used in combination with all of the other sinister ideas out there (light-up eyes, motion-sensor scary voices...), the possibilities are endless.

I live in Humboldt County, Ca., a very soggy location where everything composts in minutes, but even in this anti-pumpkin environment, the skeleton looked great for almost a week.

Of course, the decay looked fantastic, too... The entire assembly becomes a liquid mess at a certain point. It's up to you to decide just when to peel it apart and toss it in the compost bin.

On a side note, one free-hanging skull I made dried out like a shrunken head, and lasted long past Easter. Could the whole skeleton be dried out like that too? Perhaps someone (who lives in a drier location) should try that...

Happy Carving!

Mike