I'm sorry I don't know of any tutorials, it'd be much easier than writing what follows!
Even if you don't want to use an established plugin, you can learn much by seeing how other coders do things. It may be worth taking a look at the source code of Tara's linked plugins.
As it happens, adding text below the logo is fairly simple. You just need to hook the 'login_message' filter and return what ever HTML you want displayed, it is immediately echoed out by the filter. My hook reference is targeted for writing a plugin, but it also applies to customizing your theme as well. For themes, the code would go in functions.php.
To protect your theme customizations, create a child theme. For that matter, Writing a Plugin isn't necessarily any more difficult. However, if you end up with a custom login page, which involves a custom page template, stick with a child theme. Custom page templates in a plugin get sort of complicated.
If text above the logo is really that important, and jQuery is unacceptable, you will need to create a custom login page. The Codex section on this could definitely be improved. What it is suggesting is you create a custom page template to act as the login page. The template must include a call to
wp_login_form(). You can add any code and HTML above and below this. The same filters available within the form are available as they are on wp-login.php.
Once the template is created, create a page based on this template the same as you would create any other WP page. You do not need to add any content on the page edit screen, the purpose here is merely to assign a slug to the page so we can link to it.
One glaring omission is how to get your custom page used in place of the default. I've not done this myself, so I may be missing something, but I would add a rewrite rule to .htaccess to route all requests for wp-login.php to my custom page.
Hopefully that's enough to get you started. If you get stuck somewhere or something just doesn't make sense, feel free to come back here and ask. Good luck!