3-3 Ban Phase in the tournament lobby chat.
Players take turns banning 3 champions each in the on-site Challengermode lobby chat starting with the left/top player. None of the banned champions are allowed.
Step-by-step:
The leftmost/top player gets to ban first.
Ban by writing the name of the champion you wish to ban from the upcoming game.
The next player does the same thing.
Both now take turns banning one champion each until both have banned 3 each (for a total of 6).
You can start the game together with your opponent.
Note: A player is allowed to not ban any champions at all - this player has to state that he/she does not want to ban any champions in the on-site Challengermode lobby chat in that case. Starting the game without communicating this to your opponent can get you disqualified.
Note #2: If a player decides to write all three bans in one go, that is alright - as it does not put the opponent at any disadvantage. The other player may then write the names of the three champions he/she would like to ban.
Example:
Anyone that plays with a banned champion (as agreed in the Challengermode chat pre-game) may get disqualified. This means that neither of the players' banned champions can be played with (none of the 6 are allowed).
Only bans made in the Challengermode chat are valid. Written champion names are final. Any bans written after Champion Select has started are not valid.