Adding a CSS for blog

We are trying to add a blog to our school’s website, which was put together by our web developer. When we asked him about adding a blog to our website, he said this would be the best course of action to take:

Add a custom styles stylesheet:
– Make a blank file called “custom.css”
– Add the reference to custom.css to the “head” php file below the reference to “main.min.css”

I am totally lost, but would like to try to figure this out on my own, if possible. I tried going to Appearance > Editor and see the existing style.css stylsheet, but do not see how to add a new one.