I guess I’m in a talkative mood, so I’m going to explain the steps I have been using to make a history for my Cultivator Universe.
1) I started by drawing myself a galaxy, and then I gridded it off into squares. (Fortunately for me, spiral galaxies are relatively flat, so it wasn’t too huge of a stretch to use a 2D map for this exercise.)
2) I then mostly randomly created 200 dots representing the worlds on which my civilizations started. (I randomize by rolling dice. I married an avid board-gamer and there are always dice around.)
3) I randomly assigned a Civilization Level to each of those dots. These levels told me which of those planets started developing technology the soonest, and therefore who had a “head start” when it came to spreading out among the stars.
4) I made an arbitrary guestimate of how fast the “reach” civilization would expand once its ships had achieved near light-speed capabilities, and based on how much “coverage” I hoped to achieve, decided how long ago my oldest civilizations started expanding.
5) I created a technological advancement chart that would give me a rough estimate of how long it would take each Civilization Level to achieve various technological benchmarks.
6) I split my total timespan into segments, and created map overlays that showed the “reach” of each civilization at the end of each segment in the form of transparent colored circles.
7) And then, checking each overlay one at a time, I have been figuring out which civilization encountered which other civilization in what order, and writing a quick one sentence explanation of what happened when they did.