From Shuushuu

Favorites

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:

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Click the "Add as Favorite" button.

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And then you'll end up here:

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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.