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.