Hello to all!

[shemandr]

There are many ways to rig a topping lift. They normally don’t need the mechanical advantage a block could offer.
Rgranger’s diagram is for a topping lift that is adjustable at the forward end of the boom. So there would be two turning blocks to turn the TL at the aft end of the boom, and one turning block at the forward end of the boom to turn the TL down to be made fast in some manner.
The simplest is just a line (Or wire) attached at the mast head run to the end of the boom. The terminus there usually allows for adjustment of the length of the TL so that the boom height can be adjusted. Shortened when you want the boom higher as when you are not sailing. Longer when sailing so that only the sail is holding the boom up.
One my H356 the TL ran from the end of the boom up to the mast head where it went over a sheave and down to a cleat on the side of the mast. It was adjustable at the mast.

 

[jssailem]

You may ask, why is the TL adjustable?

When the sail is raised, you want the sail to hang on the boom. The boom should sit at 90º to the mast.

This facilitates the proper shape of the sail when the trim controls are being used. If the TL is taut, the end of the boom may be above level. The sail shape will be distorted. You will not get optimum performance.

As @shemandr shared, you can rig the TL in a way that fits you and your boat best.

I would say most often, the TL is holding the boom out of the way of the cockpit when the sail is down. When the Sail is raised, the TL is slack.