After all players have decided not to take any more mulligans, players choose whether to put Leylines and/or Gemstone Caverns onto the battlefield, starting with the player who will be going first and proceeding in turn order. Each player may choose any number of Leylines and/or Gemstone Caverns. The cards are revealed and put onto the battlefield at the same time.

Your “opening hand” is the hand of cards you decide to start the game with after taking any mulligans.

After all players have decided not to take any more mulligans, if the starting player has any cards that allow actions to be taken with them from a player’s opening hand, that player may take the actions of any of those cards. The starting player repeats this, taking all actions he or she wishes to, one at a time. Then each other player in turn order may do the same. After each player has exercised his or her option to take actions from his or her opening hand, the first turn of the game begins.

If multiple Gemstone Caverns are put onto the battlefield under a single player’s control before the game begins, the “legend rule” won’t put the extras into that player’s graveyard until just before the first player gets priority during his or her first upkeep step. There’s no opportunity to tap the extras for mana.