Creating Plugin to create custom login process

I am trying to build a plugin that will create its own login page and use a slightly adapted login process (adding extra steps to the standard process). Anyone know of any documentation?

I have found tons of articles on customizing the login page via plugins. This is not what i’m trying to do.

I also found several articles on creating a custom login page as part of a theme. I need this to be part of a plugin and not a theme. I also need to add steps to the login process so that tasks are done at the same time.

==== Our Problem:
We maintain a number of WordPress sites using various themes, styles, plugins, etc. They are all different. We have a database separate from our sites that maintains data on users of all of our sites (registered users). Unfortunately, the data for each user is only accessible through a special web interface on the data server. Our users had to log in to use the data from our WP websites, and then separately log in to the data site to review and update their data. It’s a messy model. The data site now allows single sign-on (a back-end login). To use this functionality, we need to make sure that, when a user registers with one of our WordPress sites, they also register with our data site, keeping usernames in sync between the two sites. I understand how to do this with the WP functions for user creation, but i don’t know how to build that process from a new page supplied by a plugin. I would like to have a plugin that i can add to our WP sites that will alter the registration process, allowing us to send details to the data site as the user is created on the WP site. Any suggestions?