Many of us RCQ grinders are getting ready to re-enter the world of Modern from some time apart, and for fans of the format it is a welcome return. In my area, there's tons of Modern to play, but not always tournament modern. And rather than focus on the actual tournament, I'd like to focus on the minutia of preparing for a tournament.
This is the most important part of tournament preparation. It effectively dictates how your go about the rest of your preparation. You need to allow yourself adequate time to practice your deck and its scenarios before locking yourself in. If adequate time for you is something like a day, then be my guest. For me, it's more like two weeks, so I lock in my deck choice well before the actual event starts.
You can hold off on submitting your exact 75, but I highly recommend having your deck's main gameplan mapped out before then. Good reasons for holding off on submitting include final tuning and card availability, but lock in your main deck idea before then and change around cards as needed for the most optimal list. One-ofs gain much more importance in open decklist tournaments, since they can sometimes force your opponent to play suboptimally around a singular card that will wreck them if you have it.
An underrepresented thought in deck selection is actually liking your deck. We still play Magic to have fun and not purely for competetive edge. I generally value my enjoyment playing the games, so I will try to find an enjoyable deck whenever reasonable as my choice. For example, I find most of Standard unenjoyable unless I'm casting Slickshot Show-Off, so I'm almost locked in on playing Prowess unless Secrets of Strixhaven changes my mind.
For this particular RCQ I'm playing, since it's Modern I already own one Modern deck that's viable, so I will be playing that. Deck selection becomes much easier if you already own a deck and don't want to go through the hoops of owning another one.
Now that our deck is selected, let's talk games. Your performance is only as good as your practice, so make it count. No matter what amount of time you actually have to practice, you should try to make it as meaningful as possible. Methods I've had success with are testing with other dedicated testing partners or a team, writing thoughts and plans in downtime at work, or grinding leagues on MTGO.
The most effective method of practice I've found is dedicated playtesting against a friend or with a team. This is really useful for figuring out exactly what happens and what cards really matter in specific matchups. If you pick one deck and get to play it for a few hours against another good player on a gauntlet of decks, you will learn a lot about how to execute your deck's plan against the field. If you're able to do this a couple of times before the tournament along with putting your learning into practice in random matches, you will have put forward a strong preparation for your given tournament. It's important to stress that in this setting, your goal is to learn with your fellow Magic players. Do anything that's necessary to achieve that knowledge, whether it's ensuring you draw your perfect seven versus their perfect seven or ensuring you test a specific card in a specific scenario. In other words, you're not playing tournament Magic; you're playing whatever you need to reach a greater understanding about a matchup.
To me, writing about a deck really helps me hammer in what a deck is about and what it is trying to do. I tend to do this in private Moxfield decklists, but any word processing application like MS Word, LibreOffice, Google Docs, or Notepad will do. Whatever will help you write, use it. This writing doesn't have to be anything fancy - simple notes or textbooks will do. You're not writing for anyone other than yourself, so write what you will. Anything that can be quickly referenced while playing a match on MTGO or Magic Arena is a plus.
Grinding leagues has the most narrow use cases, so only grind if it meets your needs or if you enjoy it. Playing a mass amount of games is very helpful to get to grips with the mechanics present in your deck and the physical actions you need to take to play your deck. Questions such as When do I resolve my Leyline effects? and similar should be answered here. This method is also particularly useful for thinking through specific scenarios in the blind against a given opponent. You have the ability to sit there and take the time to answer What if my opponent has Spell Pierce? - even if you time out, you can get the answer to that question. I tend to like MTGO over Arena for this reason, since you have more time to actually sit and think as opposed to Arena's timeout system.
Every tournament has some level of packing and traveling, whether it's your local RCQ a short walk from your house or the RC that's half a continent away. Try to optimize both in the days leading up to your event. In particular, pack exactly as many Magic decks/cards as you need, plan buffer time in your travel, and don't forget to bring food and water! It doesn't have to be a full meal, but your body will need something after playing multiple rounds of Magic.
You've already done all of your prep, so...now what? Well, put that prep into good use. Take every match one at a time and don't focus too much on what might be coming. Don't get in your own head about "oh, I'm X-0 going into day two" or similar situations - that will kill your tournament. Instead, focus on what you're doing in your current match, making good gameplay decisions, and don't change your sideboard plan too much. I advise taking some break or reset after each match, like talking with friends, stepping outside for fresh air, or something else to clear your head before your next match.