In order for your Users to be able to do things on your web service - as themselves, they need to have an account. We have seen the Registration Atom being added and configured in a Separate Help resource - where your User can create their own account for you to approve. Now they need a facility to log in to the web service. This is achieved by adding a Login Atom on your site, somewhere.
This Help Resource explains how to add a Login Atom to your website, and configure it. The attached video demonstrates this in action, on a website, from adding the Login Atom type to the web page Template, through to configuring it on the website and testing it.