One of the easiest ways to use Quick grading as an option is on your assignment submissions page.
On your assignment view page, in the grading summary, there's two buttons usually, "View all submissions" and "Grade", and "View all submissions" is the one you should click on.
This then gives you a list of all the students which are expected to submit. At the bottom of the page, you should enable the tickbox for Quick grading. Once this is done, you should see a new textbox appear in the Grade column for each student. The BEST thing is you can add ALL of the grades for the visible students into these boxes in one go, or just a selection of them, whichever applies, and then just click on the "Save all quick grading changes" once, which saves a lot of clicking!
As the Quick grading tooltip mentions, there are some restrictions and recommendations so using Quick grading is explained here, hope this helps!