Call a class inside a plugin

Hello.
I'm pretty new on plugin development.
I have a calendar class I'd like to include on a grav web site.

I was successful following this tutorial:
https://learn.getgrav.org/cookbook/plugin-recipes#output-some-php-code-result-in-a-twig-template
but I've no idea how to use my custom class inside the ExampleTwigExtension class.
When I try to initialize my class ($c = new MyCal()) inside the TwigExtension, Grav says there is no such class, even if I require_once my external file.
Of course I'm doing something wrong but I have no clue.

I have a couple of plugins that use custom classes. iplocate jumps to mind. Take a look at iplocate.php to see how it works.

Thank you perlkonig! Thank you very much!