You can create a new theory page in two ways:
When you have just created a subchapter, you can immediately create a theory page by clicking the button Create theory on the Edit subchapter page. The other way is in the Author environment.
Click on the Theory tab, and then on the New button:
You will now see a page where you can fill in some general options for your theory page:
Name: give your new theory page a name.
Position: you will see two labels and at first just one input field. For the one on the left, labelled Chapter, choose within which chapter you want to create this theory page. After that action, you will now see a second input field, labelled Subchapter. Here, choose within which subchapter you want to create this theory page.
If you started creating the theory page using the button in the Edit subchapter page, then these fields are already pre-filled.
Status: do you want to publish your theory page already? Publishing it means that your theory page can be used by teachers, and subsequently by students. Is it just a draft? You can leave it unpublished, but remember to publish it when you're ready!
Click Next when you are done filling in the general options. You will now see the editor for theory pages. Read more about it here.