I use mine on the Spyderco two-step serrations.
Lay the leather slip cover on a firm surface. Set the wide end on the leather slip cover and tilt the sharpener to the left, so that it is sitting firmly on the two left bottom points. The right side will now be at approximately twenty degrees from plumb, with a narrow edge facing out. Take the serrated knife, hold the blade vertical and draw it down and back very lightly along that right edge. The bevel of the knife will ride along the corner of the sharpener, going in and out of the serrations (if your pressure is light enough to allow it). I usually alternate between a heel to tip stroke and a tip to heel stroke to sharpen both sides of the scallops and points equally. After a few strokes on the bevel side, run the flat side down the other side of the sharpener to take the burr off (assuming there is one).
When I first heard about sharpening serrations this way, I though it would round off the points. After I bought a couple of old Police models with the points pretty well rounded off and tried this, I found that they are the last part to be sharpened, and rather than being rounded off they became more pointed.
If you prefer locking your knife in a vise and sharpening one scallop at a time, there is a radiused side on the Duckfoot that fits the wide scallops quite well, while the narrow corner works well for the narrow scallops. I just don't have the patience and skill to do it that way. I wind up convexing the scallops and rounding the points when I try.
