In FMA, the being that became Father, the one known originally as Homunculus or the Dwarf in the Flask, seemed to have been created by Van Hohenheim's master using some of Van Hohenheim's blood. Is the creation ever explained in any greater detail?
The creation is never explained in excruciating detail. However, we do know that Homunculus was originally part of the entity within the gate (God, essentially, the being who regulates all alchemical exchanges). Through an unknown process, Van Hohenheim's master was able to isolate the knowledge within God.
He was able to give it an existence through the blood of a human (Van Hohenheim), which I speculate was used as the toll to reach into God's domain. However, this existence was not stable enough to take form, and thus cannot exist outside of its flask.
Unfortunately, we hear little from Hohenheim's master, and none of it relates to Homunculus. There is never a further elaboration on the process that is used to isolate part of within the Gate.