This is one of the more time intensive victories in FE, probably only second to the Master Quest.
As usual for a quick victory setup, play on a Tiny map with 1 novice opponent. Unless you want to try for some boss mob kills along the way, set Wildlands to None. Production Pace to Very Fast, but keep Pacing at Normal or Epic -- you don't want to run out of turns while trying to meet the prerequisites. Set Magic Strength and Resource Frequency to Dense. Make sure the Spell of Making victory condition is turned on. Might want to turn off all the others (note: you can never disable conquest).
Once in game, you should work towards researching the Book of Mastery technology, which is at the end of the Magic tree. Along the way you'll unlock the ability to build improvements on shards, and you need to seek out and acquire at least one of each elemental type: Fire, Earth, Water, and Air. Then you will need to build three towers: Witch, Sword, and Padars. Once they are built, you can build the Forge of the Overlord. Expect the other faction(s) to get quite concerned with you at this point - you'll see a -9 relations hit just saying "Forge of the Overlord!" ... apparently they aren't too enthused with your plans to magically make them comply. At this point you'll need to start casting the Spell of Making. It will take 10 turns to complete, and you'll win when it is done. Expect the other factions to throw everything at you at this point, so hopefully you took my advice to only have 1 novice faction.
Congratulations on becoming the magical dictator of the world!