Where is the setting to set up role/person who can Approve/Reject documents?

This is the procedure:

https://support.office.com/en-us/article/Approve-or-reject-items-or-files-in-a-site-list-or-library-d5931756-5315-4c4a-b89f-18947b6f7305

BUT IT DOESN’T tell you how to assign people to this role to approve or reject! I did it before but I can’t remember where the heck the setting is. It is so hard to find it. Like a needle in a haystack.

This image is how the document looks Pending in the library:

Pending.bmp

This image is how the person can approve or reject the document.

Approve.bmp