How to edit a Nook or Kindle book for highlighting text?

Anyone familiar with how to edit a Nook or Kindle book? For only my own personal use I'd like to highlight certain words. I'm thinking I'd have to do something like this: 1) Download the book. 2) Convert it to something like a PDF file. 3) Use a PDF editor to highlight words then save the file. 4) Read the book via the saved filed with a PDF reader. Any help much appreciated.