The favorites on pixiv can be a little difficult to deal with. The first thing to learn is how to add something as a favorite, so we'll start off with the image page:
Click the "Add as Favorite" button.
And then you'll end up here:
But if you want to now go see all of the images you have favorited, go to the main page and then press the "Favorites" button I mentioned on the main page of the tutorial. It's called ブックマーク管理.
And that's all there is to know about adding favorites on pixiv.