Master Deliverer Achievement
Deliver 50 ghosts.
Before attempting to lead a ghost to an expiator, make sure you have an unbroken platform under the machine, with walls on either side that go up at least three blocks. This is to prevent the ghost from leaving the area before you have a chance to click the expiator. Note that ghosts, like other mobs, are able to navigate across kitty-corner blocks and through openings that are one-block high.
Mining a bloodstone tombstone gives a chance of spawning a friendly ghost. Another way to generate ghosts is to make your way to the surface of the world and bury skeletons as if for the undertaker achievement. Occasionally a friendly ghost will spawn, which you can then lead to the expiator.
Friendly ghosts will happily follow you as long as you don't move too far away from them. They're somewhat prone to wandering off if you stand still, though, whereupon they immediately seek the quickest way to fall down a hole to an inconvenient spot.
It's much easier to lead a ghost downward to an expiator than it is to lead one up, as they are only able to navigate up steps that are two blocks high, and have a tendency to randomly turn around for no apparent reason and quickly skip down all the steps you so painstakingly just led them up.
Once you've gotten the ghost to follow you to the expiator, simply click or tap on the expiator to deliver the lost soul. Ghosts must be within about five blocks of the bottom left block of the expiator for the machine to register their presence and deliver them properly.
You'll get XP for each ghost delivered. If you lead a crowd of them to the machine and deliver them all with one click, you'll only receive XP for having delivered one ghost. The amount of XP you get varies depending on your ranking in Order of the Moon. The base amount is 25 XP per deliverance.