Clamp in vice. Use triangular file to sharpen. Get a new medium sized file, don't remember the number, file is in barn and I don't want to walk there in cold right now. Take saw to harware store and get one that fits teeth. Keep file level, keep same angle as some sharp teeth on saw. Sharp teeth can be found near the handle. Saw set is hopefully still made by Stanley and you can get a new one, they're not too expensive, or didn't use to be. Or you can use a small punch to carefully bend the teeth over in line with the good teeth. The saw should be jointed, or checked to see if the teeth are in line and that there are no dips in the line of teeth. To joint the saw, take a flat file and lay it flat on top of the teeth. If there is a serious dip, the cure is to file the teeth flat and cut new teeth, resetting them.
Lyle can describe the procedure for sharpening a circular sawmill saw.