If you mean the timing of the sickles, you disconnect both drive shafts from the header. extend them separately to the extreme outside of the throw, then reattach the couplers. There should be a disc inside one both of the large shields opposite the attach point to the header with letters on it. Once you have the sickles timed you loosen an allen screw in one of the discs and turn it so the letters are the same in both windows.