How do theists think God was created?
They don't: theists believe God is "out of the time constricts".
Being eternal (and omniscient) is not the same as living forever or knowing everything: God is uncreated because creation implies developing in time and God is outside of the time.
From the point of view of God, the Universe is all in front of him, he can see you at a glance from the time you were born until you die.
Actually that is not a very good presentation of the vision that God has of the Universe: He does not "glance", because a glance is an act in time, He simply encompasses everything.
You cannot say things like "God knows everything from the beginning", because in a God's view there is no beginning: to begin is a human concept, a concept of beings embedded in time.
I am myself an atheist, but to ask how God was created makes no sense: time itself is a creation of God.
God is not a "part" of a Universe that is ticking in time, an Universe that has processes that cannot be described outside of time.
God encompasses everything, including the time you exist into.
If you want to know a bit more about the theoretical attributes of God (of course, as much as humans can comprehend them) I highly recommend you study a bit of Scholasticism or Kabbalah for a comprehensive treatment of the "the relationship between God, the unchanging, eternal, and mysterious Ein Sof (אֵין סוֹף, The Infinite), and the mortal and finite universe (God's creation)".