The block secured to the lower end of the tye is the fly-block.
Luck is like a fly-block and its yard: when one goes up, the other comes down.
Have the fly-block of the topsail halyards above the top, and rack the runner to the topmast backstay or after shroud.
Then make fast your hook to the bight of the rope, and reeve the other end through the fly-block for a fall.
Reeve the end of your rope through the upper block, and make it fast to the strap of the fly-block.
The halyards should be a luff purchase, the fly-block being the double block, and the single block being hooked in the channels.