How to make a fixed menu when scrolling the page?

Question

css

The question is how to make a fixed menu when scrolling? it is desirable to manage the css of the webasyst admin.
I would like to do as in this example
rso.marketingup.ru

Answer 1

Is it really that hard to ask the right question in the title?
What should be the IQ to fail to form an interrogative sentence? Double digit?
Although what I'm talking about: solid text without punctuation marks. There is not even an example that is mentioned in the text.
The diagnosis must be made.
UPD: thanks for understanding