Starting a battle
Summons can be used to have battles, and this is the primary way they are used to gain an advantage over your opponent. To do this, summons must use up their "action." Take a ready summon you control and turn it on its side to act with it.
After you act with a summon you control, you then select your attack target. A summon can attack any other summon, including your own! It is not recommended to attack your own summons all of the time, but you may find very specific cases where you need to though.
After you select the attack target, the battle begins! Compare the strength values of the two summons that are battling. For example, a summon with 5 strength battles a summon with 3 strength. The smaller one will take 3 damage (because that is the most amount of damage it can take), and then the larger one will also take 3 damage. The winner would be left with 2 strength, and the loser would be defeated and sent to its controller's graveyard. The winner will be left with 2 strength forever, until its strength changes again through an ability or another battle.
During your main phase, as long as you can pay the cost, you may declare attacks as often as you want. Attacks are declared one at a time, and it is important to give your opponent a chance to use any abilities they may have when you attack to avoid being rude on accident.
Declaring direct attacks
While summons are able to declare attacks against each other, they are also able to attack players directly to deal damage to them. To declare a direct attack, the column of the attacking summon must be open. You cannot attack a player who has not started a turn, but players are able to attack themselves directly!
(You can ignore wizards when direct attacking. Think of them as being up in the castle with you, looking down on the battle occurring below them)
If nothing interferes, the defending player will take damage to their life equal to the attacking summon's strength. However, the defending player can take a summon in an adjacent column and move it over to block the direct attack. This causes the target of the battle to change to the defending summon, and then a normal battle proceeds. If a direct attack is declared, but the only summon the defending player has is two columns away, it won't be able to block the direct attack or move.
Remember: A summon can attack any other summon, but in order to attack a player using a direct attack, the column needs to be open.
Some cards have abilities that occur during a battle. Those abilities need to occur at the correct time, so this flowchart will help you determine when to use those abilities. These are the states of a battle, along with examples of ability conditions that are met at that time.
Pay the cost to attack. (When this acts)
Declare that you are attacking, and select the target of the attack. (When this declares an attack) (When this selects an attack target)
If the target is a player, the player may block the direct attack with a summon in an adjacent column. If they do, the attack target is changed. (When this is attacked) (When this blocks a direct attack)
Determine how much damage will be dealt and which summon will win. (When this would take damage) (When this would lose a battle) (When this would be defeated)
Calculate damage dealt to each summon.
Determine the winner of the battle. (If there is a tie of strength, both are considered losers, and both will be defeated. If neither summon is defeated, neither are considered winners or losers.)
Deal the damage. (When this is dealt damage)
Take the damage. If damage dealt is 500, and the target of the damage has 2 strength, the damage taken will be 2. (When this takes damage)
Defeat any summons with 0 strength. (When this is defeated) (After this takes damage)
Move the defeated to its controller's graveyard. (After this is defeated)
If strengths change during step 4, such as through an ability of a card, repeat step 4 again. If a summon is defeated during a battle through an ability, the damage dealt in step 5 misses.
Remember that bosses have two special rules when it comes to battling.
They may block direct attacks from any column, and do not move when doing so.
They can only attack directly if the target's entire side of the battlefield is clear. (For a side of the battlefield to be clear, the boss is either face-down or not on the battlefield, and they have no summons)