Name your class, add a description, select "Study Group" or "Class" and a color. This will allow you to share the flexbook with your students when you are finished adding content.
Start adding items to your library - anything can be part of your flexbook. Use the "Add to CK-12" from the Library Tab to put bring in outside content.
You can also add vetted content from their bank of items to your library. Once you have all your content in your library you can create your flexbook and send it to your class.
Back on the dashboard start your Flexbook! You can upload your own documents - word, google, or content you have placed in your library from CK-12 or from the internet.