This is an excerpt from the combat rules page of my rulebook. One of my 'things' is that the GM keeps a deck of playing cards in front of them, this gets used for all sorts of things.
When combat starts, the PCs act before NPCs. If two PCs are fighting each other, then the one with fewer Hit Points acts first - if they're tied then their actions are simultaneous.
In some combat scenarios, the GM may want to give the PCs a challenging foe who acts before them, or a large group of enemies whose actions happen throughout the round. For these, use a shuffled deck of cards.
Each NPC combatant gets a single card draw, and each PC draws a number of cards equal to 1 plus their DEX, minimum of 1 card. Once everyone has their cards, the GM counts down using the card values starting with Ace then King, Queen, Jack, 10, and so on. For tied cards, PC actions happen before NPC actions. Players still only act once, but high-DEX characters get more chances to act earlier in the round.
Combat is measured in rounds, each round being roughly 6 seconds long. During a round, each character takes a turn fulfilling some sort of action. When all characters involved have attempted an action, the round ends, and if the conflict needs to continue then a new round begins.
During their turn, a character may move and commit to an action. Actions can include making an attack, pulling gear from a pocket and using it, casting a spell, activating a magic item, or some other activity that requires attention and care.
Some actions are called Free Actions, and these can be performed at any time, usually in reaction to something else that is happening.
A player can also choose to Hold their action to act later in a round. A player may hold their action until the end of the round, effectively acting last, or they can use it to interrupt an NPC's action, or to act in tandem with another player. A player cannot interrupt another PC's action, they happen simultaneously or based on who has fewer Hit Points.
To attack, the player rolls 1d20 and adds their Attack score and the attribute bonus associated with the weapon being used, either STR or DEX unless the weapon is magical. If the total is equal or greater than the target's Armor, the attack hits. Circumstances can apply other penalties or bonuses to the Attack roll.
If an attack hits, the player then rolls the damage dice associated with the weapon and adds the appropriate attribute bonus. This damage is then subtracted from the victim's Hit Points. NPCs reduced to zero Hit Points are either dead or mortally wounded.
Hit Point damage represents the wearing down of a target’s stamina and energy. Only the last handful of hit points represents an injury that does serious physical damage.
Being attacked is similar to making an attack. The player rolls 1d20 and adds their Armor score. Opponents always have their own Attack score and the player needs to roll equal to or higher than their opponent's Attack score to evade, dodge, or block the attack. A player may also choose to Defend as their action. If they do this, their Armor score is +4 until the next round.
When a PC gets hit, they take damage just like when they make an attack. When a PC is reduced to zero Hit Points they fall unconscious and must make a Constitution saving throw or die. Similarly, if a PC suffers more than half their total Hit Points in a single blow they must make a Constitution saving throw or die.
The PCs have the potential to make a Critical hit when they attack. This happens in one of two ways: when a player rolls a result 10 points higher than what they needed to hit with their Attack roll, and when a player rolls a 20 on their Attack roll - the only exception being that when a player needs to roll a 20 to hit, no Critical is possible.