How to make a fixed (floating) horizontal menu when scrolling?

Question

css

Good afternoon :)
Guys, I ask for help.

I am doing a coursework. It is necessary to create a textbook on telecommunications computer networks.

Downloaded the template. Things are good. There is a horizontal menu. It needs to be fixed so that it "floats" along with the scroll.
We need such a result . So far, this is just what happened

to me . Here is the "textbook"

Answer 1

https://www.google.ru/search?newwindow=1&site=&sou...