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.