Congratulations! You are now the Chapter Master of a Space Marine Chapter, Commander of an Imperial Guard Regiment, Canoness of an Order of Sisters of Battle, or what have you. The first thing you should do is decide where your army comes from. If you are stuck for a name, you could use the name of the street or town you live in with your house or apartment number appended. So if you live at 12 Delancey Street, Charlesport, New York your army homeworld can be Delancey XII or Charlesport XII.
If you have an idea of what sort of army you want to build, you can name the planet accordingly. If, for example, you are going to build an army based on Hindu iconography, naming the homeworld something like Veda or Hind makes sense. An army based on a Jewish theme might hail from New Judea or Lyubov. A Celtic force might come from An Tir or Albion or Galacia or Gael. You get the idea. Choosing a homeworld’s name goes a long way to establishing your army’s theme and personality.
Once you settle on a name, you can go into as much or as little detail of the homeworld as you like. You can content yourself with a thumbnail sketch of the world (a jungle death world filled with dense jungle, large predators, deadly insects, and lethal diseases) or a detailed map, weather charts, cultural details, or something in between.
GW official regiments generally have only a thumbnail sketch (which you can flesh out if you like). I opted for the detailed homeworld route while many very successful army designers have gotten by on the middle ground. Decide what sort of climatic and cultural conditions prevail. Frank Herbert’s Dune was memorable for its odd water customs, giant sandworms, desert tribes, and so on rendering the setting a complete whole. No one is suggesting you write a science fiction epic but you might want to create a solid backdrop for your army to suggest what sort of units will (or, as importantly, will NOT) be present in your forces.
Lyubov is slightly smaller than Earth, possessed of a cooler and more severe climate. Horse-mounted Chazak rough riders are very likely, and I can use standard Atillan models. Ogryns are converted from Fantasy Ogres to maintain that ‘Caucus Mountains’ look that is appropriate to an army that is inspired by a cross between the Kievan Rus, the Jewish Shtetls, and modern Eastern Europe.
Knowing something about where your army is from makes these decisions easy. It also helps to define your forces in terms of color and background. You can approach this from either direction. You can either derive your army from your homeworld (I have long toyed with the idea of creating an army based on GDW’s Tarsus supplement) or create your homeworld to fit the fluff for your regiment. Both approaches are equally valid.
You can get as detailed as you want with your homeworld. Most official GW regiments have only a paragraph or two that describes their homeworld. Valhalla, for example, is described as an ice world covered in ice and snow year-round. Leaving aside the scientific implausibility of such a world, Valhalla does explain many aspects of the Valhallan regiments. I wanted my regiment’s home to be more plausible and better fleshed out (it’s the Traveller referee in me, what can I say?).
After getting a map, I figured out the likely local conditions (thanks to GDWs old Scouts supplement for the Traveller RPG.) I also looked at the old GDW Animal Encounters supplement to give me some ideas for what the local fauna might be like.