Step 1: Slide the fly up the line and keep away from the knot.
Step 2: Form the first part of the knot as shown and pull this part tight leaving an open loop at the end.
Step 3: Slide the fly down the line to the knot and slip it through the loop. Hold the eye between the fingers and pull on the main-line closing the loop at the eye of the hook.
Step 4: The completed knot allows the fly to swim parallel and straight to the mainline. With other knots the fly often sits at an odd angle relevant to the mainline putting fish off the final take.