We've covered the importance of giving feedback in Book Creator in a separate article - check that out if you haven't already.
To get started, go the + button then Apps and select Feedback Stickers to enable it. You can choose to enable it just for you, or you can select which libraries you want to allow it for your students too. You can also access and enable Apps from the Teacher Dashboard. On the top of the screen, you will see the tabs for Libraries, Discover, Learn, Certification, and Apps. Simply go to the Apps section and enable or disable the apps of your choice.
Note: you will need a premium subscription to be able to add the Feedback Sticker Pack to your books.
Once enabled, you can explore all of the Feedback Stickers by scrolling down or using the navigation icons at the top of the list. When you've chosen the one you want, click on it or drag it onto the page.
Once it's added, you can reposition or resize it just like any other image. You can also add hyperlinks and alt text with the Inspector (i button).