The question asked for a tutorial which the other answers I don't feel cover,
one can be found at:
http://www.codeproject.com/Articles/403355/Implementing-Two-Factor-Authentication-in-ASP-NET
The tutorial was written by Rick Bassham and covers information on:
"What is Two Factor Authentication" "What is Google Authenticator" "How does it work"
It then explains how to implement code for:
"Counter Based One Time Password Generation" "Time Based One Time Password Generation"
And gives a full tutorial using Visual Studio 2010 under:
"How do I put it to use"
