Can you guys recommend a good tutorial on responsive subnavigation?

As you can see here, I am trying to build a centered responsive navigation menu.

This menu contains a submenu, and if you test it out in mobile view, you can see that it is… not very good (sub navigation floating over the main navigation).
I have been looking like crazy for some good basic tutorials.
w3schools is sadly not useful, because their example is completely different from how I set up my HTML code, so it will only make me more confused if I try to copy/paste from them.

When I look on Youtube I mostly get overly flashy menus or videos with techno music blasting in my ear while someone is speed-coding.

Do you know of any good tutorials that would match my current setup?
All I want is to arrange the mobile-navigation so that when you press one of the dropdown links, the dropdown menu will push the other items down