I've been working on this side project of mine developed in ASP.Net MVC 5.
While developing the project, I started with an empty MVC project template.
As a result of this, the default plumbing about authentication etc was not part of this project.
I was now trying to figure out a way to add authentication support to it on the lines of forms authentication in ASP.Net web projects.
I came across an excellent post on this topic by Ben Foster, in whereby he explained the process of adding cookie-based authentication to an ASP.Net MVC application in a step-by-step manner.
It's a two-part post and I highly encourage you to check it out here