Thread the loop of your leader through the loop on one end of the sink tip. Slide the sink tip's loop a few inches up the length of the leader.
Thread the free end of the sink tip through the leader's loop. Pull it tight to secure the loop-to-loop connection.
Do the same with the loop at the other end of the sink tip and the loop on the butt end of the fly line.
Overlap the ends of the leader and sink tip by approximately 12 inches. If one of the two ends has a loop, pass the unlooped line through the loop to a depth of 12 inches instead.
Cross one end over the other and pull it beneath at a right angle, leaving only a 3-inch overlap. Lay a 3-inch hollow straw next to the overlapped lines.
Wrap the crossed line around the straw, spiraling up its length until you've made a total of six turns. Thread the free end of the line through the straw.
Slide the straw out of the knot and pull tight. Trim the loose ends.
Do the same with the other end of the sink tip and the butt end of the fly line.