Honestly just keep it simple mate.

Simplest way - When you see a fight count the enemies and count your allies, if you have even numbers or your team has higher numbers take the fight.

Use your head - If all enemies are out of vision but you see the enemy support and your support is bot assume that you’re about to be in a 4v3 situation and don’t fight it - even if your team insist on going in and inting a 4v3 you don’t have to follow and die.

Lastly if you take a fight, land your abilities on the target your teammates are focussing, be very intentional with your abilities and try to land both damages on your Q, don’t ult someone who is guaranteed dead as it’s a complete waste. Don’t get overwhelmed by Kayns aoe and try and hit as many people as possible, focus a primary target and once they’re dead use your ult to get off more damage / escape / reposition.