Call a class inside a plugin

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:
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!