Theres a page that details what it takes to mirror.
The process of “mirroring” consists of taking an exact copy of the Tranquility database, including character and account data, like a snapshot. This copy is then processed by the CCP Deployment team and all starbases are removed in order to prevent players from spying on their positions and then using this information on the live server. Basic moon minerals are available in all moons, to help testing and prevent moon scouting on the test server.
All accounts are edited so they don’t expire. Standing and Loyalty Points are removed. Science and Industry jobs are removed (because jobs in starbases would get stuck). All assets of structures are moved into asset safety. The market is seeded so that all market items cost 100 ISK and are in plentiful supply. At least one station per constellation is market seeded like this. In addition there are some other internal adjustments. This entire process can take up to 3 days, then the Singularity server is open for public
Characters created in between mirrors will not be able to get on SiSi until after the next mirror is completed