This week I had some time at lunch during a training to write out ideas for the worlds in my Psi-Marine campaign. I wrote 5 pages of ideas, 1 page for each of the the major factions, and 1 page outlining the data that I want to cover for each faction entry. Keeping things to one page really helped me to concentrate my ideas into the most important information. It also helped me to contain my creativity. For example, I written that there are 22 systems in the United Colonies. I've only written about the 9 most important. I don't have any idea of what the additional 13 colonies are like but I now have an idea of the range of culture within the colonies, I have solid ground to stand on for future improvisation, and finally, I have given wiggle room for my players to add a colony/colonies to be the homes for their characters if they would like to participate in the world building.
The funny thing to me is that when I typed out my one page of notes for the United Colonies, it ended up being 5 pages! I was getting a little worried about overloading my players until I realized a couple of things. First, this world building is primarily for my benefit. The first section in the campaign guide is titled, "Campaign Background: The minimum reading to not frustrate the GM" and that is only 3.25 pages long. Second, I know that the United Colonies entry is going to be the longest and each successive entry will be shorter. The United Colonies are the home for the PCs so this makes a lot of sense to me.
Here is an outline of what I included in this entry.
Basic Information: This paragraph includes a bit of history and ties the United Colonies into the arc of human history as described in the aforementioned "Campaign Background" section.
Psions: This section describes the history of the United Colonies in relation to Psions, which in this campaign are fairly new in human history.
Resources: This includes those vital things for space exploration, water, food, energy, human knowledge, etc. I rated each resource according to Fudge's 7 basic attributes, made a brief one sentence note as necessary on 2 of the resources.
Important Colonies: Each Colony has a 1-2 paragraph entry describing, physical composition, cultural differences, major resources (how it contributes to the United Colonies resources already listed), and human knowledge that it specializes in. If there was anything important about that colony in relation to Psions, I also listed that in a sentence or two.
For those of you who are also writing sci-fi campaigns I found the following resource very helpful but didn't bind myself to accurately reproducing the information here:
I aimed for a variety of colonization methods. I have cloud planets like Venus, rocky planets like Earth and Mars, moons circling Gas giants, sulfuric acid atmospheres, and one water planet. I have one asteroid colony in another faction too. I mean hey, what's a sci-fi world if you aren't mixing things up a bit.
This is what I used to build my GURPS fantasy campaign, I think you can use the same system to build a fantasy world as well. Countries are the fantasy equivalent of the major factions (i.e., United Colonies would be the home country of the PCs). Regions are the fantasy equivalent of a star system. Cities are the fantasy equivalent of the colonies that I've described here.