How was The Dwarf in the Flask created? In the Fullmetal Alchemist Brotherhood, it was revealed that 


   The Dwarf in the Flask was an experiment of the King of Xerxes and that it has Van Hohenheim's blood. 


But it wasn't explained what method was used to create him. So, how was The Dwarf in the Flask created? Was it ever explained in the manga?
The backstory of van Hohenheim and Father is discussed in chapter 74 and 75. All we see is this:


The Dwarf in the Flask talks to slave no. 23, whom he names.
Van Hohenheim is taught to read and write, and ends up in a better position in the household.
The king wants immortality, and the homunculus tricks him and destroys the whole country, excepting Hohenheim.


The only thing relevant in these chapters to how the homunculus was created is this: it notes that blood was taken from Hohenheim to create the homunculus.

From chapter 74:



The notion that they have blood relation is repeated later in chapter 75, but this is the main thing we ever get as to the origin of the Dwarf in the Flask.

It's possible of course that there might have been more information in bonus material to the series, but the fact that the Wikia lacks anything beyond this and that I've never seen any serious commentary on this among fans of FMA suggest that this probably isn't the case.