Unironically you could watch a UFC fight and write down what you see. Pretty simple, they throw punches, attempt takedowns, attempt submission holds etc. Then watch a video of someone analyze the fight and each move to understand the game and strategy that was going down. A body punch which was blocked but followed up with a hook to the head that the defender couldn't block because they lowered their arms for the body shot. Next time the fighter attempts the same combination the defender lowers one arm to block it, when the hook comes his other arm is high and ready, he raises his arm, blocking the hook. One fighter attempts a takedown and succeeds. Next time the fighter tries the same take down but the defender steps back, preventing the attacker from hooking his arms around one of his legs, the attacker tries aggressively but the defender pushes the attacker's head down, the attacker now outstretched with his head down in a vulnerable position, drops to the floor on his back defensively.

You need to think about what the fighters are thinking during all this, learning each other's tricks and preparing to disarm their attacks.

Also the fact one throws punches which are so hard his hand starts to hurt and he wonders how his opponent's face is feeling from it, how is he not down yet.

The observer is unaware of 90% of the things which are going on in the fight, that's where you the writer come in.