The basic steps include:

  1. Prepare your docx file: First, using the Styles menu (not the formatting menu) in Word, make sure you apply NORMAL styling to the entire document; and apply Heading1 style to your chapter headings.
  2. In Pressbooks, go to: Left Menu/Tools —> Import.
  3. Select: File Type: docx
  4. Choose your file from your computer
  5. Upload
  6. Select which content/sections you want to import
  7. Decide whether the content/sections are: front-matter, chapters, or back-matter
  8. Import
  9. Arrange your chapters in the right order
  10. Test some exports, clean up markup, and off you go!

The Pressbooks User Guide provides more details, including tips for formatting your Word document prior to importing, and screenshots. It also provides a link to a YouTube tutorial video which may be useful to you.