601. Casting Spells
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. [...] To cast a spell, a player follows the steps listed below, in order.
601.2a [...] propose the casting of a spell, [...]
[...]
601.2f The player determines the total cost of the spell. [...] Some spells have additional or alternative costs. [like] discarding cards, [...]
601.2h The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid.
601.2i Once the steps described in 601.2a–h are completed, [...] then the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, they get priority.