In your academy, you need to have researched "colony ship".
You need 170 open farm spaces
You need the proper building permits from your local Building, Zone & Code Enforcement Department.
Remember, all this will allow you to do is start a NEW city. If you want to conquer an existing city.... and WHO doesn't... You need your academy at level 28+ and you need to also have researched "conquest".
I completed first conquest successfully in Xi. Battle results showed no casualties-there were no enemy units. After receiving message that conquest was successful I recalled that whole attack force, but everything came back except CS- and it's not showing as being in conquered city. Where did it go?