The best way to figure out how to implement it with 4.6.x and later is to simply look at the code that is used for one of the core providers.

You can get the AD source at http://dnn.gmss.org. The source files are in the .zip in a file called ActiveDirectory Source.resources. Just rename the extension to .zip and you can get to the files. Or install it on your dev system under Host->Host Settings->Authentication Services. The AD source is probably more information than you really need.

You can also look at the source for the DNN login as the .vb files are already there under DesktopModules/AuthenticationServices/DNN.

Basically, a login provider is just another project like any module so you can code it as such. In the database there's a table called Authentication that if you look at the entries are pretty self-explanitory and you'll have to fill the information concerning your provider manually while you're developing. Also instead of building your .dll into the bin directory it has to go into the bin/providers directory. The last thing is I believe the .dnn file is slightly different.