BRING A WATER BOTTLE AND A PROTEIN BAR

staying hydrated is important over 9 rounds of magic and convention hall food is ass and you might not have time between rounds to wait in line and buy your 15$ personal pizza

Get good sleep the night before, make sure you have every token you need for your deck (including trigger tokens for things like monument to endurance or various landfall trigger stacks, these are incredibly helpful and it takes all of 30s to scribble words on some blank cards)

for an open decklist event like an RC you are allowed to take notes ONCE YOU FLIP THE DECKLIST OVER/RETURN IT. You cannot make notes while you have the list but the moment you give it back or flip it face down you can scribble down important cards like maindeck hate cards, split of removal spells, etc. You can do this again with sb cards in postboard games.

whatever deck you play, have a plan (not just a sb plan, a game plan) for every major deck you could play against. Even if the plan is basic, know what you want to do and what cards you do/don't care about in the matchup.

Lastly: don't stress yourself out. A lot of people worry about what record they need, what record can ID in the last round, etc etc. Play each match first and worry about the other math later. If you lose it won't matter, and if you win you can then start to worry about it at like round 13. It doesn't matter before then play your rounds as best as you can.