Step 1: Materials
You'll also need some duct tape, scotch tape, paper mache glue (I use a flour and water mixture. It's cheap and strong, but won't be weatherproof until you seal it), and paint (I used a can of light brown spray paint for the base, and some dark brown acrylic for details).
Step 2: Making Bones
Fold the arm tubes and leg tubes in half. Then, using your own limbs as reference, fold over the extra paper on the thin side and tape it down.
Step 3: More Bones
Tape the ribs to another tube that will act as the spine (in some of my more recent skeletons, I have used a piece of 3/4" pvc pipe for a spine, as it is much stronger and can even be bent into a slight "S" shape with a heat gun (as a real spine would appear). However you form it, beef up the base of the spine with some more tubes. How long should you make it? Use yourself as reference.
Those circular squiggley things are for the pelvis. There are more accurate ways to form it, however, since most people won't know the difference, this will suffice.
Step 4: Knobbly Bits
Then, cut a piece of cardboard for that thing in the middle of the ribs. Tape it to all of the ribs, front and back. If you want, you can also fashion some triangular-shaped shoulder blades from cardboard.
Step 5: Paper Mache
Step 6: Painting
With some dark brown acrylic, sponge on the texture all over. You can cheat, if you know at what angle people will be viewing the final prop.
At this point, you're probably wondering what we're going to do about the skull. My advice: just buy one of those four dollar foam skulls and hot glue it on. It's just not worth your time to try to make a realistic skull from scratch. Of course, you can always embellish it.
You'll also want some way to keep those bones together. I drill a small hole into the end of each one and glue a piece of wire in, connecting the two bones together.