Depending on how bad it has been treated, I use a draw file. I clamp one handle in a vice and use my chest to support the other handle. From there, I lay the draw file on the bevel, make sure it is flat, and push/pull the file along the blade. That will remove material from the bevel in order to take out nicks and such. Once the bevel has been re-established, I use a much finer stone to finish things out. I lay the stone on the bevel again, then use a circular polishing action to sharpen. Once the whole blade has been sharpened, I flip it over and use the stone to true up the flat side. Alternating back and forth between the bevel and the flat with the fine stone with diminishing amounts of pressure until I have no wire edge. From there, it takes very little more than stropping with leather glued to a flat piece of stone to keep a fine edge.
Some folks actually prefer a dull edge to their drawknives. Not me.