Rather than pay {2} for each previous time you've cast this spell from the command zone this game, pay 2 life that many times.
Flying, lifelink
Whenever a player casts a spell, they lose 2 life.
Liesa's first ability applies only to the "commander tax." If there are any other additional costs to cast Liesa, those costs must be paid as normal.
Liesa's first ability isn't optional. You can't choose to pay mana to cover the "commander tax," even if you have that mana available.
Liesa's last ability will resolve before the spell that caused it to trigger.