STEP 1 Thread the leader through the hook eye and form a loop around the leader behind the fly. Pass the tag end twice through the loop, forming a double over-hand knot that also serves as a simple slip knot.
STEP 2 Hold the loop in your left hand and pull on the tag end with your right hand to tighten the double overhand knot. Now pass the fly through the loop.
STEP 3 Pull on the main leader portion to seat the knot around the fly's head right behind the hook eye, adjusting the position with your fingers as you tighten. Trim the tag end.