Its fairly simple:
Each round you will get a randomized crew of 3 characters against another player who will also receive 3 randomized characters.
Each randomized character will have a randomized haki proficiency level ( from no haki to like maxed out hakis) and a random devil fruit.
From then on a thread will be made for you and your opponent and then you and other onlookers shall engage in debates, memeing or just agenda posting to convince as many people that your crew would win the fight. If you win you advance to the next round,etc.
Here is an example of what the crew vs crew RNG looks like: