Can you tap lands for mana to pay for a spell before announcing all the targets?

When you cast a spell, the rules define a strict sequence of steps (CR 601.2). Choosing targets comes in step 601.2c (choosing modes, then targets), while paying costs — including tapping lands for mana — happens in step 601.2h. You must announce all required targets before you pay any costs.

This matters because the game needs to know the full identity of the spell (including its targets) before payment is made. You cannot use "I'll tap lands first and see how much mana I have" as a reason to defer target selection; the order is fixed by the rules.

However, there is an important practical note: mana abilities (like tapping a basic land) can be activated at any time you have priority and also during the process of paying costs (CR 605.3b). So while you declare targets before paying, you activate your mana sources during the payment step — not before targets are named.

Concrete example: You want to cast Lightning Bolt targeting your opponent's creature. You must first announce Lightning Bolt, then choose the target creature (601.2c), and only then tap a Mountain for {R} and pay the mana (601.2h). You cannot tap the Mountain, look at your {R} floating, and then decide what to target.

If a spell requires multiple targets (e.g., Electrolyze), all targets must be chosen in the target-selection step before any mana is paid for that spell's cost.