Align one tab in the menu to the right side

Hi!
How can I align one tab in the menu bar to the right side?
I read through this topic but unfortunately it didn’t work out for me or at least I did it wrong.

Okay. The easiest would be to this.

Make your menu. Then click on ‘screen options’ and check ‘CSS classes’

With that available, edit the ‘about’ menu item, and you’ll see an option for CSS Classes (optional).

In there, put become-far-right.

In your CSS use this:

li.become-far-right {
float:right!important
}

Where should I insert this CSS code? To styles.css or custom CSS or anywhere else?

Thanks for your help!