1. OCC (small world, deity) rules apply until the space ship is launched.
2. You must launch a 15 year spaceship (or less). In other words, if it helps your score, feel free to build a ship with more population, but it can't be a slow boat to China to allow more time to conquer.
3. You cannot settle or conquer a city until after the spaceship is launched. You can, however, delay the launch as long as you want while building up your military. If another civ lands a spaceship first, however, you lose.

For what its worth, my best percentage is 52%. I launched the standard 15 year ship late (mid-1900s), which made the AI cities larger and ensured that they had RR connecting them. I didn't build up as much militarily before launching as I could have because the whole idea was spontaneous. Happiness management was a pain with respect to newly conquered cities (they never had temples and I stayed in democracy to prevent ai city bribing). I have always taken the military route, but saving gold and building spies might be more effective, unless of course the target is in democracy (which they invariably go to if you bribe a city or so). Luck is a big factor, because you prefer to take large cities with wonders, which is easier if they aren't an ocean away.