This is a short video created by the National Archives explaining how they converted microfilm to a digital format and how to use the enumeration district numbers. For those of you in the digital technician program pay close attention toward the end of the video.
At the National Archives site you can get a primer on how to do a census search, the vocabulary you need to understand and more. There is even a blank 1940 Census form for your perusal.
You can get much of the information from proprietary sites like Archive.com, Family Search.org, or access to ProQuest.
This is a great opportunity to find that rascal of a relative or to get a better sense of how people functioned in the the time between the Great Depression and World War 2.