Montressor's plan is not that complicated if you think about it. He waits until Mardi Gras, when he knows Fortunato will be inebriated. He dismisses his servants, who wanted the night off anyway. He uses reverse psychology by using Luchresi and the Amontillado as bait.
Then, he plants a trowel and mortar in the catacomb. He rigs up some iron fittings (that may have been there already). And he proceeds to wall up Fortunato in the place for a perfect murder: a catacomb (where dead bodies go to die).
The key to his plan is the imaginary amontillado. It is a vintage, rare bottle of wine. It is the Mona Lisa, the Mickey Mantle rookie card, the piece-de-resistance of wine connoisseurs. All he has to do is to pretend he has it, and the rest is as easy as leading a horse to water.