The advice from Snow and Nealley is to try to follow the angle that the axe has now—15-25 degrees. The specific angle you use can depend on your intended use. Make it flatter (a thinner edge) for cutting, less so (a blunter edge) for splitting.
When sharpening, you begin with a hand file, belt sander, or grinding wheel. However, use the last two options with great caution, as they can overheat the steel and ruin the temper. Finish the job with a hand-held sharpening stone either wet or dry.