This is a pretty straightforward concept. But it needs you to primarily focus on team score rather than personal score. It also involves being watchful. I CANNOT STRESS THIS ENOUGH, STICK TOGETHER. It’s been said a million times and is a boring cliché, but people still fail at it. They either fall way too back, leaving their teammate alone, or they rush ahead instantly, rendering themselves useless.
Know what type of weapons you’re good at, and your teammates. If someone on your team is a better sniper, be kind enough to give it up. You’ll have someone covering you when you rush. Know not to throw a grenade at your opponent when your teammate is in the fight. But ultimately do what’s best for the team. If you do betray, that’s one less kill given to opponents. Any gained kills are a bonus.
Never miss opportunities. Firing at an opponent when he’s distracted by your teammate can be the best way you can help them. Stop worrying about yourself and go save some lives. Make sure your teammates have the same mentality, and you will find your team automatically winning.