I don't think there is necessarily a precise way to "relax" but I can tell you a few things I use to do as a player:
That's all I got.
Just remember you practice hard all the time to succeed in game situations. Everyone makes mistakes, just have a short memory when it comes to that. If you worry, you won't perform good at all. So, just think positive thoughts and imagine yourself doing what you do best.
Always remember, if you blow a play. So what? Even you lose the game becuz of that, it can happen!
Just try to think: I want to get this W. And if you don't, you feel disappointed. But shake it of you.
And if you have to make a play, just try to play your own game. Don't give your attention to screaming and exited people. Just go on your on rythim and try to do what you do always!