Can you tap a land for mana before paying the additional cost of a spell?

When you cast a spell, you follow a series of steps defined in CR 601.2. After announcing the spell and making all required choices (targets, modes, etc.), you reach the cost-payment step (CR 601.2h), where you pay all costs simultaneously — the mana cost, any additional costs, and any alternative costs all get paid together in one payment action.

Crucially, CR 601.2g instructs you to activate mana abilities before paying those costs. Tapping a land for mana is a mana ability (CR 605.1a), so you are explicitly permitted — and expected — to tap your lands for mana during this window, which happens before any part of the total cost is actually paid.

Additional costs (like sacrificing a permanent, discarding a card, or paying extra mana) are simply folded into the total cost you pay in CR 601.2h. They do not create a separate, earlier payment window that would prevent you from using mana abilities first.

Concrete example: You cast Fling (sacrifice a creature as an additional cost, plus pay {1}{R}). You first activate your Mountain and another land to produce {1}{R} (CR 601.2g), then in the payment step you simultaneously sacrifice the creature and pay {1}{R} (CR 601.2h). You never need to sacrifice before generating mana.

So yes — always tap your lands (and activate any other mana abilities) in the mana-ability window of CR 601.2g before committing to paying the full cost in CR 601.2h.