Historical Rules · 1 May 2011
Rule 103.2
Official source text from the 1 May 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.