606.3 A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if no player has previously activated a loyalty ability of that permanent that turn.
606.4 The cost to activate a loyalty ability of a permanent is to put on or remove from that permanent a certain number of loyalty counters, as shown by the loyalty symbol in the ability’s cost. This cost may be modified by other effects.
606.5 If the total cost to activate a loyalty ability contains multiple costs to add or remove loyalty counters, those costs are combined into a single cost to add or remove loyalty counters, as appropriate.
606.6 A loyalty ability with a negative loyalty cost, taking into account any additional costs, can’t be activated unless the permanent has at least that many loyalty counters on it.