My first homebrew is my current campaign, and I have been running it for my friends for 2 years. But I started working on it about a good 6-8 months before actually beginning to game with it.
The world is a post-Ragnarok setting with extreme climates, old Gods, new Gods, and lots of conflict. The setting was inspired by reading the Death Gate cycle, by Weis/Hickman. That series had several different worlds, separated by magic, that each had significant flaws. Well I twisted that concept and asked what would happen if several different worlds sort of came together in one, and that is what happened in my Midgard campaign. As a result of Ragnarok and the death of the Midgard Serpent, the Elemental Planes have become coexistent with the Prime Material. In each corner of the world, one element rules supreme, and between those extremes there are varying degrees of elemental mixture. In a centralized region, all four elements coexist and a relatively normal climate exists. This region uncoincidentally just happens to be the center of the most conflicts throughout the history of the New World, and where the majority of my campaign has taken place so far.
But where did I start? I began with the theme, and crudely sketched out on a legal pad how the 4 Elements existed as overlapping circles on Midgard. Then I figured how each combination of Elements would result in a certain type of terrain and climate (for example, Fire and Earth formed a very hot, volcanic region of mountains that serve as lairs to many Fire Giants and also hold a portal to Muspelheim.
Step 2 was to draw my first map of the world overall, and fill in details of the central region. Most extreme regions are to this day undetailed, but still sketched out in very general terms in my campaign notebook for future use.
Step 3 was to decide on the cosmology of the planes and the deities of the world.
Step 4 involved what races were generally available (and if any were prohibited), what role would classes (and prestige classes) have in the world.
Step 5 was to outline a general historical overview of the 18 centuries since Ragnarok.
Step 6 was to outline various major conflicts existing in the world, whether it was between races, nations, religions, organizations, etc.
Step 7 was to figure out some overarcing theme (or two) going on behind the scenes, which would somehow impact the campaign and the PCs.
and Step 8 was to figure out where to begin the adventure, and I decided to start with the free PDF adventure the Wizard's Amulet and its followup, the Crucible of Freya.
so far what I have learned is that it is a constant process of upkeep and modification. And I love it.