At the moment, there isn't an automated way to create an index in Atticus. We understand the importance of indexing for non-fiction projects, and our team is actively researching and exploring possibilities to implement this feature in a future update.
Typically, an index isn't as important for an ePub, because each individual can search the content of their book for a specific word and it will automatically jump them through your content, just like an index would. For the print version, however, if you want to create one yourself, it will be time-consuming and probably not the most fun part of creating your book, but it is possible.
This is what we would recommend:
Make sure your book is finished - you need to be sure nothing will change in either the content or the formatting at all
Once everything is 100% complete except for the index, add a new chapter at the very end of your book - it must be the very last page, or it will alter your page count.
Enter all the words or phrases you'd like indexed - you can use a list, hanging indents, and/or press CTRL or CMD SHIFT in between each entry to adjust the spacing, but you cannot create columns
Once you know all the items you want to index, click into your first chapter or page and open the Print preview - click the icon in the very bottom right and change the preview to Print
Press CTRL or CMD F to open the browser's Find feature.
Enter each word/phrase and, if it's in the chapter, click the Preview to generate a PDF of this chapter only. Find the page where the word/phrase is mentioned, and note that in your Index.
Repeat for each word/phrase, one chapter at a time.
You won't be able to search the PDF copy itself, because your content is specially embedded for print and copyright protection, which means the Search feature does not work in Atticus exported PDFs.
Again, we know this is not an easy thing to do, which is also why it's incredibly difficult to program. We absolutely understand it's important to you to create an index, and our team will continue to explore possible ways in which we can automate this within Atticus in the future, but until then, this is the only solution.
I hope it helps,