Eric about covered everything. Use caution when sanding. Depending upon the construction, it may be just a thin cosmetic veneer on the exterior. These are easy to sand through if too aggressive, especially on the corners and edges. I would begin with a 220 grit paper and see if that is enough. If not, I wouldn't go below about a 180 or so. Sometimes a few scratches are less of a problem than sanding through the veneer. I wouldn't be too concerned with sanding all the way to bare wood. If you choose a similar color stain, or darker, it should cover most imperfections in the old finish and just a light surface sanding should be enough. How old is this humidor? If it is an antique old time varnish, a different process may be in order.

If there is not a good seal at first, depending upon how dry it is, sometimes the wood will swell with the seasoning process tightening things up.