Someone at our prerelease was slightly confused because, as per the rules, we explained that you had to crew a vehicle before combat in order to declare it as an attacker. Now, this player thought the same thing applied to blockers, and attacked into a board whereupon the defending player crewed his own vehicles. How exactly does declaring attackers work - not only with vehicles, but with creatures in general?
So this gets down to the basic structure of the combat phase, and explaining that should answer this question.
The combat phase is divided into five steps:
There’s nothing special about the beginning of combat step. It’s a normal step and mostly exists for the purpose of triggered abilities or certain spells that can only be happen in this step. After any triggers go on the stack, players get priority to cast spells and activate abilities. This is the last chance for players to cast spells or activate abilities before attackers are declared. Once the stack is empty and everyone passes, the game moves on to the declare attackers step.
The first thing that happens in the declare attackers step is the declaration of attackers by the active player, as the name implies. The active player chooses which creatures they control will attack, and which opponent or opponent’s planeswalker each of those creatures will attack. Once attackers are declared (including paying any costs to attack) and any attack triggers have gone on the stack, players receive priority to cast spells or activate abilities. This is the last chance to cast spells or activate abilities before blockers are declared. Once the stack is empty and everyone passes, the game moves on to the declare blockers step. (If no creatures were declared as attackers and no creatures entered the battlefield attacking during this step, the declare blockers and combat damage steps are skipped.)
The first thing that happens in the declare blockers step is the declaration of blockers by the defending player(s). The defending player chooses which creatures they control will block any creatures that are attacking them or a planeswalker they control. Once blockers are declared (including paying any costs to block) and any blocking triggers have gone on the stack, players receive priority to cast spells or activate abilities. This is the last chance to cast spells or activate abilities before combat damage is assigned and dealt. Once the stack is empty and everyone passes, the game moves on to the combat damage step.
The first thing that happens in the combat damage step is the assignment and dealing of combat damage. First, the attacking player assigns combat damage for each attacking creature they control, then the defending player assigns combat damage for each blocking creature they control. Then all assigned damage is dealt simultaneously. Once combat damage has been assigned and dealt, players receive priority to cast spells and activate abilities. When the stack is empty and everyone passes, the game moves on to the end of combat step.*
(*If one or more attacking or blocking creatures has first strike or double strike when the combat damage step begins, only creatures with first strike or double strike will assign and deal combat damage in this step. After this step, there will be another combat damage step where combat damage will be assigned and dealt by creatures with double strike and any attacking or blocking creature that didn’t assign/deal combat damage in the first combat damage step.)
The end of combat step is a normal step without any special turn-based actions. Some spells or abilities can only be cast at this time and some abilities trigger at this time. After any triggers go on the stack, players receive priority to cast spells and activate abilities. This is the last opportunity to use and spells or abilities that refer to attacking or blocking creatures to any actual effect. Once the stack is empty and everyone passes, the combat phase ends and attacking or blocking creatures cease to be attacking or blocking creatures.