Remember that the purpose of the AEM Companion is to provide content that could not be included in the book that will expand readers' awareness and knowledge. Thus, it should not simply repeat content in the book but should add to it in a significant way. This means either including works that are not in the book or writing/showing more about a work that is in the book. Since your entries will usually be about timed-based media, including multimedia content, especially videos, helps make the material come alive. The content should be as accurate and objective as possible, like Wikipedia.
Format: In general, I like entries to start with an image or video at the very top with text beside it. This provides the best appearance when new entries are promoted to the front page.
Citations: Be sure to properly cite your sources, using quotes "..." for direct quotations of others' texts and clearly indicate when you are using the ideas of others. For consistency, and in order not to clog up your narratives with citations, please use footnotes, just as you would do in an academic paper. Insert a footnote reference number in brackets (eg.  in the text itself that clearly indicates what specific material has been quoted or derived from another source, and at the bottom of the page, put the same footnote reference number with the course cited. If citing an online source, make the URL a link.
Editing: GUI editing screen is very intuitive in many ways, particularly for writing texts and linking to images, positioning images in the text, and creating links. It is not intuitive for adding video. Please see the following section, Images and Video, or more details. Another problem is that text copied from an online source often will become strangely formatted (described below). For those of you who are familiar with HTML, there is also an HTML editing window option, which will allow you to compose entries. The GUI editor is not perfect! So please don't get overly frustrated if it does not behave properly. Rather, please register a comment, and hopefully your solution, in the GUI/HTML Editor forum.
Text: Cutting and pasting text inserts formatting that ends up with strange results. In order to avoid that, cut the desired text from an online source, then paste it into a SimpleText editor, like TextEdit. This removes funny formatting. Next, copy the text from TextEdit into the Companion editing window. Voila!