If the organizer has match check-in turned on:

Once you have been seeded into a bracket you will get a match notification. Press the Go to Match button on it and you will be taken directly to your match page.

Once you are on your match page you will see an "I'm Ready" button and a timer. Pressing this will set you as ready in the match and alert your opponent that you are ready. If your opponent does not ready up before the timer runs out then the system will automatically give them a loss and move you forward. The same applies to you, so make sure to press the button before the timer runs out.

Note that you will need to check in for every match that you play while match check in is enabled, so keep an eye on Battlefy for your notification.