But if none of those things concern you, then please read on.
FIRST: Make sure you have permits to possess whatever critter you want to skeletonize. Most states require permits for skeletal material--even for species that are extremely commonly found as roadkill, like raccoons.
OK. Now that you either (a) have a permit to legally retain the skeleton of your specimen or (b) determined that a permit is not required [double-check federal, state, and local regulations!], then you can begin.
1.) Flense the carcass as thoroughly as possible. This means removing the skin, muscle, and viscera using knives, scalpels, or scissors. The more work you put into it now, the cleaner the skeleton will be in the end. Disarticulate sections of the carcass if you want to conserve on box space (see below).
2.) Find or build a box of suitable size. "Suitable" here is a sliding scale--if you disarticulate the carcass, you can get a relatively large animal into a relatively small box. If you want to arrange the carcass in "anatomical position" (see the Steller sea lion layout), that takes substantially more room.
3.) Fill the bottom third or so of the box with a sediment of your choice. Clean paving sand works great, but so does horse manure, potting soil, or even just plain dirt. Be advised, though, that you want the grain size of the sediment to be smaller than the smallest bones you expect to encounter. I learned the hard way, for instance, that there are a lot of faux-bones (sticks, twigs, rocks) mixed into horse manure. Sure, the skeletons come out looking great. But you run the risk of losing small bones.
4.) Best to leave your box outside, open to the elements. The sediment should stay moist, but not wet. It also works best if the air temperature is 60 deg F or warmer. I have heard of several people leaving containers inside for the duration of the burial, with little or no offensive odors. But this approach should probably only be used as a last resort.
5.) Depending on the size of the critter, the average air temperature, and several other factors [like how much material you removed in Step 1], your critter might be done in as little as a week, or as long as 6-8 months. I have dug things up that were not flensed that even after two years were not very close to being "done."
Why not just bury the thing in the ground? Yes, I suppose that would work. But I prefer to use boxes for a variety of reasons.
A.) when I started experimenting with this approach to cleaning skeletons, I was a renter. Consequently, I did not want to chunk a bunch of holes in my landlady's yard. I also was never quite sure when I might be moving to a new place, and I didn't want to have to leave anything behind.....
B.) earth is a tremendous heat sink, and has an average year-round temperature of 50 degrees or so. Critters decompose much faster if they are in warmer conditions--above 60 deg F is ideal.
C.) things buried in a yard or field are really really easy to lose.