Search for a which class you'd like to favorite.
Tap the white outlined star icon in the top right-hand corner of the class, it will then turn to a full white star icon.
Once you favorite a class, it will be added to the favorites section at the top of the categories page for you to view. If you do not see your favorited class at the top of the categories listed right after favoriting it, then go out and back into the Reflect app. you will then see it.