Here are the changes I needed to make to add a login system to my existing ASP.NET MVC 4 project:
1) Upgrade to MVC 4.5 from MVC4.
2) Change all references to ApplicationDbContext to instead refer to [MyDbContext] (since it was not named ApplicationDbContext).
3) Edit MyDbContext to have a "create" method (which just returns a new MyDbContext).
4) Edit MyDbContext<ApplicationUser> to extend IdentityDbContext<ApplicationUser> instead of DbContext.
5) Edit constructor for MyDbContext to extend : base("name=[connectionStringName]"). EF can no longer create a database for you, you must create the database manually (just create an empty one) every time you change the tables and want to recreate it.
6) Edit ApplicationUser (which extends IdentityUser, and is found in IdentityModels.cs) to add additional fields, foreign keys etc etc.
7) Do NOT add ApplicationUser to DbSet<...>. This will break things. (ASP.NET Identity already has a Users DbSet which refers to it.)