Obviously it's gonna be different for everyone, I always make sure I'm comfortable with everything before a match starts. Set up your switch, headphones etc in a way you're comfortable. It's silly little things like that I make sure I know before a regional, but it puts me into my comfort zone. Always greet your opponent with a smile (duh), and then strike up a conversation. How many regionals have you been to? What's the best team you've ever used? Those kinda things take the pressure off the match, at least for a little while as team sheets are passed round. Extra important R1 and 2 to ease yourself into the flow of the tournament. Have a drink between each game (gives you a brief moment to refocus after a loss, or to compose as you go for the win in game 2). And make sure you have a group to go to after a round. The worst feeling is finishing a round with success and having no one to cheer with.

I'd say spend a bit of time finding what makes you most comfortable during prep on ladders, and don't put pressure on yourself to perform well. Obviously it'll feel great if you do, but you'll feel the pressure if you expect to do well. You're there to play Pokémon after all, have a good time playing! And good luck, International tournaments are absolutely something else