1.  Start by making a comprehensive multi-year calendar for all the dates covered by your fiction. 

http://personal.ecu.edu/mccartyr/MyCalendar.html

2.  Spend way too much time putting it together in photoshop. Fill in every important date both historically and for each character personally.

3. If you suddenly realize they will be outside looking up at the sky at any point at night for a significant reason, look up a retroactive star chart, enter their specific latitude and longitude points and the hour of the day in order to accurately render any star formations. 

4. Decide which general direction your characters will be facing.

5. Kinda go crazy a little while panicking about minutia.

6. Cry a lot.

7. Now you are done in 7 easy steps.

Next time: Furniture and Settings!! (I don’t care how many 1915 San Francisco building floor plans you’ve found, you will never be accurate enough and you should just quit while you’re behind \o/)