Once a decklist is created via the Your Decklists page, players can assign a decklist to a tournament by: Your Decklists > Green cog icon on the left-hand-side > Use for Future Tournament > select a tournament from the drop-down menu.

Once a decklist has been assigned to a future tournament, the Player Controller will open showing that decklist. The player can then finalize the submission by pressing Submit Decklist.

See Decklist Submission for more details.