Instructions
Learn Currency technology. It costs 80 gold. This will become available after your civilization has learned the Bronze Working and Code-of-Laws technologies.
Collect 60 gold and build a market.
Choose to have your civilization learn Literacy. It costs 90 gold. This will unlock Banking in technologies.
Earn 190 gold and learn Banking.
Collect 120 gold and build a bank. It is available in the Building menu.