tldr: You can find everything you need before and during the tournament on your DASHBOARD!
All tournament matches, unless otherwise explicitly stated, are played through the official Pokémon Trading Card Game Online (PTCGO). All info on how to download and play the game can be found on the Pokémon website.
Create an account
Before you can register for a tournament, you must first create an account on this website. In addition to the obvious required information, you can also enter your name and country, which you can choose to use in tournaments instead of your username. You can also edit those later in your profile settings.
Browse the tournament overview to find one you want to play in and follow the highlighted registration link. You will have to enter the screen name of the PTCGO account you want to use in the tournament, and if your account has profile information set, you can also choose to customize how you appear on standings and pairings. Click on register and you're already done!
Once you are registered, you will find the tournament listed in your dashboard. Your dashboard is the only place you need to visit for all your tournament related tasks. You might also see important reminders or links on the tournament pages themselves, but you can always find all you need in the dashboard.
If you want to change any of the data you registered with or leave the tournament, you can do so until the registration closes by following the link in your dashboard.
Most tournaments will require you to submit a decklist to participate. Follow the link in your dashboard and enter your decklist in text form. It is recommended to export it directly from PTCGO, but you can also use other formats, like copy/pasting it from the rk9 labs decklist creator.
You can re-submit and change your list as often as you like until the submission deadline. Usually it will be open until the tournament starts, but some tournaments might close it a bit earlier. You can take a look at the tournament's Schedule page to confirm when decklists are due.
Players that don't submit a decklist will be automatically dropped from the tournament once it starts and can not be added back.
Tournaments may require you to check-in prior to the tournament to confirm attendance prior to round 1. It usually opens an hour before the start of the tournament. To check in, simply go to either your dashboard or any of the tournament's pages and click on the highlighted check-in button. You can find out when exactly check-in starts in the tournament's schedule.
Players that don't check in will be dropped from the tournament, but can still check in after it started and receive a loss for every round they missed.
During the tournament
Your dashboard will automatically update and let you know whenever you got a new match, so please keep it open at all times! If you enable notifications in the user menu in the top right, the dashboard will also send a notification to your device whenever it gets updated. You can check when rounds are supposed to start in the tournament's schedule if you want to know when exactly you need to be back for an upcoming round.
Playing a match
If you click on a match in your dashboard you will be taken to the match page on which you find all your opponent's information like their PTCGO name and their decklist (if decklists are public in the tournament). Send a friend request to your opponent and then start the match once both of you are ready.
To report a game, click on either the Win or Loss button. The selected result will be automatically accepted after 15 seconds if your opponent does not report the game themselves, or is instantly verified if both player's report the same outcome. If you selected wrong and want to cancel your result submission, click on the active button a second time.
If you got any issues with your match, you can call a judge into the chat and explain the situation there. They can reset matches and report games if needed.
Don't forget to read the rules for more details on how the matches are to be played!