Some of the resources that I used:

"Singleton Design Patterns"
-- http://csharpindepth.com/articles/general/singleton.aspx
-- https://www.youtube.com/watch?v=sbML3xFHRbI
-- https://www.youtube.com/watch?v=oFphNlh5l2Y
-- https://www.youtube.com/playlist?list=PL6n9fhu94yhUbctIoxoVTrklN3LMwTCmd