Basically download whatever brushes you want to, and (in most cases) there'll be a readme explaining how to do it. I'm going to post it here in simpler terms.
First of all, make sure you're not using the trial of SAI since it'll be really dumb and not allow you to bring in files.
So, usually when you download brushes (or any kind of large folders/downloads with multiple files) it'll come in a .zip folder. If you don't have winrar already, I suggest you download it. Drag the .zip folder into wherever you keep SAI, and if it asks you if you want to move the files, say yes.
Make sure you moved it to the correct folder, or else it won't work. Close whatever you have open, then re-open SAI, and your brushes still might be there. Depending on which brushes you downloaded, the brushes themselves might be there, or in the .zip file there might be some more files, this time with the brush settings. If that's the case, you're going to need to right click an empty brush spot and make a new one, then fix the settings identical to the others.
tumblr / 내 호버크라프트는 장어로 가득 차 있어요. ㅡㅅㅡ
thank u elliot for the pic of me n my gf♥