605.3 Activating an activated mana ability follows the rules for activating any other activated ability (see rule 602.2), with the following exceptions:
605.3a
A player may activate an activated mana ability whenever they have priority, whenever they are casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment, even if it’s in the middle of casting or resolving a spell or activating or resolving an ability.
605.3b
An activated mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after it is activated. (See rule 405.6c.)
(refs: 405.6c)
605.3c
Once a player begins to activate a mana ability, that ability can’t be activated again until it has resolved.