Humans have skeletons inside them, but they are also muscle, tendons, skin, among other things. Lucky for your skeleton, it doesn't need muscles or guts. Also lucky for your skeleton, muscles and guts are easy to duplicate! Let's assume your skeleton has seen people at a distance, enough to know how they act, look, etc.
After some basic observation, move on to some up-close examination of a human, via grave-robbing! A little knife-work will show how humans look under all that skin, and give insight into the next step.
Using leather stuffed with wool or cotton, the skeleton makes muscles, and using strings as tendons, ties them to itself. A little balled up cotton or wool, and he has guts, too. Next, layers of cloth, wrapped around the "muscles" and bones, to simulate fat. Since it will all be under a layer of clothes, the skeleton's work could be fairly clumsy and still pass muster.
The last step is most difficult. Shoes and clothes can hide everything but the skeleton's face and hands; depending on how it dresses, it might even be able to wear gloves. Its head, however, is still obviously a bleached skull. A mask of some sort, even just a handkerchief over the mouth and nose (maybe with some clay to form features), and a hood would work for a little while. The skeleton is ready to go exploring - at least a little bit.
It should start exploring in winter, while everyone is bundled up; using whatever treasure the skeleton has collected, it should go to town, walk in like it owned the place, and pick out a lovely wig. If possible, it should find some fake eyes, and any other disguise-worthy materials, including makeup.
With disguise in hand, the skeleton can make itself appear entirely human, apart from one small detail: its face. This is where its fake eyes come in. Using clay and a little tanned animal skin, make a face - doesn't have to be a good face, just vaguely lifelike. Using the makeup, give the face a horribly scarred appearance, like a burn victim. Finally, don a simple mask.
Now, the skeleton looks and feels human, and can explain why it wears a mask: due to an accident, it suffered terribly burns from fire/acid/lightning/whatever, and is forced to hide its ugly skin under this mask.
The longer it spends with humans, the more it learns, and eventually, it may find someone willing to create a lifelike skin for it. Without very complex mechanical tools, it will never have the muscles to smile, blink, breathe, or wiggle its eyebrows, but it will eventually be able to pass for human. As long as no one looks too hard.