How do I create perfectly round buttons?

I am trying to find a way to create circle buttons on my website, but I am having a hard time knowing where to put the custom CSS (and what to put in the first place). I'm a complete beginner so any help would be appreciated! 

CSS goes in Design -> Custom CSS

border-radius: 50%;

Should give you the look that you posted, but you will need to get the button element's names. If you post a link to your page I can get that for you.