Most MtG formats are defined by the legality of the sets and cards which are allowed to be used. 3-Card Blind usually follows a similar principle and a banlist is provided by the organizer.
In this Metashaping event, the initial banlist will consist only of conspiracy, ante and un-cards. Each Round, the top 4 finishers will have a chance to ban a card from their deck for the next Rounds. Once 50 cards have been banned, the top 4 will instead replace one of the older bans.

You can see the current banlist here.

If registration is open, just submit your deck; make sure you don't use any of the banned cards. Registration is usually open for 5 to 7 days every 15 days, so just come back next week if it is not open yet!

After all decks have been submitted, you will be assigned a group. You have to calculate how your deck performs against the other decks in your group and then submit your results. You will have about a week to do so.

If you have any questions just hit me up on Reddit at u/Lognu.