Any Tips On Building An Underground Fallout Shelter?

Me and a friend are planning to hand build an underground fallout shelter. Any tips on how to build it would be appreciated. We plan to dig at least 12 feet underground and we will have concrete walls and floors and at least two rooms.one for storage and one for living in. Also we are taking requests for roommates during the next nuclear holocaust. we accept rent in installments of canned food or more preferably help building it or providing building supplies.