custom css file

Hi, I’m very new to WordPress. I am building a site with the Buddypress, Frisco theme and am trying to insert my own title logo.

I’ve seen other posts about this and it says about creating a new custom css folder in the edit directory. Sorry but where and how do you create this new folder?

I get to the theme editor and there is no clear option to create a new file. Can someone please just explain in simpleton terms how to do this.

Thanks.