This tutorial is for adding the button in navigation. If you want a button in a menu navigation of your site and also want to give styling like background color, hover color, border etc to your button. Just follow the tutorial, you can do this by adding some simple custom CSS to it.
Just follow the instructions given below:-
First go to Appearance > Menus
Create a menu name, select page and custom link to the menu. ( Page > Add new)
Adding a class id for your page just select the CSS class option in menu structure item and save it.
This will add a button to your site and you can also add a link to it.
Now go to customize and add custom CSS by inspecting the menu item id follow this link
Here are some examples for designing the button using Custom CSS.