[Captain Kwok]

It's basically the same as SE:IV in a way. Take an existing AI set ([EmpireName]_Main_Script.txt, [EmpireName]_Setup_Script.txt; located in the Utilities\SE5EmpireScripts folder) and make whatever changes to the script you'd like. This might be changing what weapons they use, perhaps the racial traits they pick, or different percent values for diplomacy. You'd then use the Script Parser to compile the two files and viola, you have your custom AI files.