To sharpen a serrated blade you should first obtain the correct sharpening tools and have the proper technique. Many of the sharpening kits on the market offer serration hones as options.
Most factory ground serrations will have the same angle as the plain edge portion, usually around 20 to 25 degrees, assuming the blade is partially serrated.
Once you have everything set up you can begin the sharpening process. Using firm pressure, work the hone in a back and forth motion, perpendicular to the cutting edge. Every so often, stop and feel for a raised burr on the backside of the blade. You should move on to the next tooth only when you see or feel a raised burr. Once you have completed sharpening the ground side of the blade, flip the knife over and begin again.