Historical Rules · 17 June 2011
Rule 103.2
Official source text from the 17 June 2011 Rules release
After the decks have been shuffled, the players determine which one of them will choose who takes the first turn. In the first game of a match (including a single-game match), the players may use any mutually agreeable method (flipping a coin, rolling dice, etc.) to do so. In a match of several games, the loser of the previous game chooses who takes the first turn. If the previous game was a draw, the player who made the choice in that game makes the choice in this game. The player chosen to take the first turn is the starting player.