1) Relax
2) Relax
3) Make sure you actually keep correct score(moves list)
To my eyes, G/90 is still somewhat fast for what a real chess game should be, but I find a lot of players around here never use their time wisely.
What I would do is go through the opening theory that you know is 100% correct fairly rapidly, say 2 minutes for your first 5 moves. Then, as you get out of book, slow yourself down and come up with a plan.
It doesn't really matter how long it takes you to find a plan, even a 15 minute think isn't the end of the world here. Once you do come up with your goals and do the calculations, the next few moves should be fairly quick since you only need to pause and do a quick tactical check to make sure your analysis didn't miss anything.
That's all kind of advanced, though. Really, just sit on your hands(literally!) so you don't move too fast. if an average game is ~40 moves, then you have over 2 minutes to find each move, and if you throw in 10 or so forced moves into that sequence you'll see that there is a LOT of time to play your middlegame and endgame, if it get that far.
I would just make sure I'm aware of the rules for OTB play, and try to get some practice games in at my local club with a clock beforehand.
PS. Do lots of tactics the week before the tournament. I mean LOTS.
I've been playing 25 minutes games at my local club and I am thinking of joining a tournament in another area of town for the first time. I've never played in a long official game such as a 90 minutes. The tournament lasts 3 days so I was wondering how people play such games. In other words, it will be my first time that I will actually track my moves and my opponents on paper and force myself to think longer before moving. Any advice before going to such a tournament.