How to Sharpen Serrated Knives
To sharpen serrated knives, you can use a sharpening rod to individually sharpen each groove on your blade. Like the rods, a triangle-shaped sharpener works great on serrated knives, especially those with V-shaped gullets. If you prefer to make your own tool, you can buy some dowels, wrap them in emery cloth, and sharpen your knife with this homemade kit.
Purchase a serrated knife sharpening tool. Serrated knives need different sharpeners than straight-edged blades. Most serrated knife sharpeners are rod-shaped, usually with a taper to accommodate different sized serrations. Locate the side of your knife with a beveled edge. Serrated knives generally do not look the same from both sides. On one side, the face of the blade will continue at the same angle until the blade's edge. On the other, the face of the blade will angle down slightly just before the serrated edge; this is called the bevel. Only apply a sharpening tool to the beveled edge. Place the sharpening rod in one of the serrated scallops ("gullets"). Choosing an angle is easier for serrated blades, because you can use the angle of the bevel as a guide. This is typically between 13 and 17 degrees compared to the edge of the blade, which is shallower than you may be used to sharpening knives. If the knife also has a straight edge portion, the bevels are usually ground to the same angle — about 20 to 25 degrees. If you want a better guide, draw on the gullets with permanent marker. You'll know you're hitting them at the right angle if the marker is removed. Move the rod to match the gullet's diameter. If your sharpening rod is tapered, locate the rod in the gullet at the point where the rod's diameter is the same size as the gullet's, or slightly smaller. Sharpen the first gullet. Run the sharpening rod along the first groove in several short strokes. Push in one direction away from the edge of the blade, toward the spine. Rotate the rod as you push it for a more even grind. Push only to the point of the rod with the same diameter as the gullet, to avoid enlarging the gullet. Check for a burr. Run your fingers along the backside of the groove to check for a "burr," or metal shavings. As soon as you can feel a burr, you have sharpened the groove sufficiently. This often only takes a few strokes. Try running your fingernail along the back edge. If you feel it catch, there's a burr. Continue sharpening each groove on the blade. If the knife's serrations are different sizes, adjust the position of the tapered sharpening rod so that the rod just fills the groove. File away all the burrs. The burrs are the metal shavings that you filed off when sharpening the blade. To remove them, rub the back of the knife against a sheet of fine-grit sandpaper. As an alternative, you could lightly run the sharpening rod against the backside of each groove, being careful not to apply more pressure than necessary to remove the shavings. Sharpen any straight-edged portion of the blade. If your knife is only serrated along a part of the blade's length, sharpen the remaining length with a whetstone or other sharpening tool. Don't attempt to use the serrated knife sharpener on the straight portion of the blade. Use a triangle-shaped sharpener. Like the rods, these tapered triangles are designed specifically for serrated knives. Due to the shape, they tend to work best on knives with V-shaped gullets. The process is almost identical to the rod-shaped method described above, except that you rock the edge of the tool back and forth along the bevel instead of rotating it. Make your own tool from emery cloth and dowels. If you would rather not buy a single-purpose tool, you can buy a few cheap dowels from a hardware store instead. Find a dowel that sits in the knife's first scallop without wobbling, then wrap a piece of emery cloth most of the way around the dowel. Hold the cloth in place with your finger, and sharpen carefully and slowly. Switch dowels to match the size of the scallops as you move along the knife. Use a round dowel for a knife with round scallops, or a square dowel for V-shaped scallops. Sharpen with a square stone. This is a difficult and unreliable last resort, but it is possible. Hold the knife spine securely against a hard surface and tilt the blade so the beveled edge is on top. Bring the corner of the sharpening stone to the scalloped edge and use it to sharpen the edge, rocking it back and forth to cover the full area of each scallop. 
Purchase a serrated knife sharpening tool. Locate the side of your knife with a beveled edge. Place the sharpening rod in one of the serrated scallops ("gullets"). Move the rod to match the gullet's diameter. Sharpen the first gullet. Check for a burr. Continue sharpening each groove on the blade. File away all the burrs. Sharpen any straight-edged portion of the blade. Use a triangle-shaped sharpener. Make your own tool from emery cloth and dowels. Sharpen with a square stone. 