andycatts

{}=amount of mana in your mana pool

Add 2 mana from any source other than the Monolith you intend to use in the combo. {2}
Tap Monolith {5}
Spend 3 to put Monolith's untap ability on the stack {2}
Pay 2 to copy the untap ability {0}
Copy resolves, Monolith untaps, tap it as a mana source before the original ability resolves {3}
Original ability resolves, Monolith untaps. {3}
Tap Monolith {6}
Spend 3 to put Monolith's untap ability on the stack {3}
Pay 2 to copy the untap ability {1}
Copy resolves, Monolith untaps, tap it as a mana source before the original ability resolves {4}
Original ability resolves, Monolith untaps. {4} No extra mana needed.

Remember that as soon as you have the ability to pay the untap and the copy you will net one mana. Since you started with 2 from another source you will end with 3, you don't magically lose that two mana you injected from outside the combo...

It's a pauper deck. Uncommon commander (doesn't have to be legendary) with common everything else.

diregraf captain