To set up a large print theme, to either click “Create New Theme” or use the three dots next to a theme title to edit an existing theme.
Under Chapter Heading: to align with the suggested guidelines, you'll want to pick a Sans Serif font for all headings, and ensure your headings are left-aligned.
Under Paragraph settings, choose Spaced.
Under Print Layout, make sure to uncheck Justified and Hyphens.
Since your large print book will likely contain more pages than the original version, you'll want to make sure that your Inside margin is large enough to accommodate the new page count (check with your publisher or preferred distributor for Gutter/Inside margin requirements based on the page count).
Under Typography, check the Large Print box— your font settings will transform to meet the recommended standards. The Body font will be set to Large Print Open Sans (no italics) and the font size/line spacing will be updated.
You might also consider a larger trim size to accommodate the larger text and better manage the final page count.
If you'd like to save the Large Print theme separately from your main theme, you can click Save as New Theme at the top of the page when you're done.
After your theme is saved, you can apply this same theme to any of your books and you'll immediately have a Large Print version!