You may pay the only once per spell.

If you cast a creature with bloodthirst while Leyline of Lightning is on the battlefield, you may use the Leyline's ability to deal damage to an opponent. If you do, that damage is dealt before the creature spell resolves, and the creature will enter the battlefield with counters from its bloodthirst ability.

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 Leylines in his or her hand, he or she may put any or all of them onto the battlefield. Then each other player in turn order may do the same.

Leylines that start the game on the battlefield aren't cast as spells. They can't be countered.

After each player has exercised his or her option to put Leylines onto the battlefield, the first turn of the game begins.