By using [th-favourite] and [toggle-favourite]!  I’ll give you how I did it but with the essentials so you can stylise it yourself! >wo)9

First, you would have an a class of the following—this establishes that this group acts as a favourite button (leave the href blank):
<a href="#" data-toggle="th-favorite">

Then, inside that, you would have TWO icons, one for the favourite button and one for the unfavourite button (I based mine on Toyhouse’s fav button but you can do whatever, as long as it’s distinct so that people know):

<i class="favorite toggle-favorite far fa-star"></i>
<i class="unfavorite toggle-favorite fas fa-star"></i>

And that’s it!  So it should look like
<a>
    <i></i>
    <i></i>
</a>

I haven’t tested this out so I’m not sure if it will work 100% but that’s how I did it.  Anyways, hope that helps a little!!