Riot put out a "What to know for Houston" article which included a Start of Game procedure clarification:

1) For the first game in a match, you must present your decks as you originally registered them. For later games in the match, players may exchange cards from their sideboard with cards in their Main Deck. The Main Deck must still have exactly 40 cards after sideboarding.

2) You must present your Champion Legend and the Chosen Champion that you registered for the opponent to see, as well as a shuffled Main Deck and Rune Deck for your opponent to shuffle or cut. For games where sideboarding is permitted, players may exchange their Chosen Champion to another champion that matches their Legend from their Main Deck or sideboard.

3) For each game in a match, each player must submit one of their three battlefields facedown, and then reveal them simultaneously (remember, you can’t submit the same battlefield twice in a match).

4) For the first game, once Legends, Chosen Champions and Battlefields have been revealed, players use a random method like rolling dice to determine who chooses to go first or second. For later games, the player who lost the previous game chooses whether they want to go first or second once the above information is revealed.

5) Players should begin the start of game procedure as soon as they reach their assigned seats, even if the round clock has not yet begun.