I'm lazy and this is just the basics. I'll be updating it later so keep checking.
This instructable is for realistic extraterrestrial design. Star wars/trek have mostly got it wrong- too much like earth. This instructable is for creating things like the daggerwrist or skewer from Wayne Barlowe's Expedition. Remember the number one rule: Don't make things too much like earth
This step speaks for itself. Just come up with details that could effect life- heat, geography, climate, gravity, air composition, and so on. Remember that this is what the lifeforms are based on; if you give the planet a low gravity there's going to be a lot of flying things.