Add a custom FTL-page

As an example, let’s say I want to add a custom new page where people can check if they have created an account already. A simple form where they can enter a username/email that checks if that already exists in our db.

How would I do this?
I have changed the theme and altered the existing FTL-files quite a few times already, but I would now like to add a new page and can’t seem to figure out how. This new page should also use ‘template.ftl’ as layout.

Thnx!

No one?
By new page I mean a new ftl-page. So ‘special-form.ftl’ that is ‘inserted’ into the template.ftl page.
Why is this so hard?