How do I import highlights from documents I sent to my Kindle?

Your Kindle device or Kindle app only syncs highlights to the cloud from ebooks that you purchased directly from Amazon. You can see those highlights here: Kindle: Your Notes and Highlights. These are the highlights that the Readwise extension can automatically synchronize for you.
For highlights from books, articles, PDFs, and personal documents not purchased from Amazon, you can use a different technique to import your highlights to Readwise depending on whether you're reading on a Kindle device or a Kindle app.

Kindle Device (My Clippings)

Every highlight you take on a Kindle device is added to a file called My Clippings.txt saved on the Kindle device. You can extract this My Clippings file by plugging it into your computer using USB. Then you can email or upload it to Readwise and all those highlights will be imported into your Readwise account.

Kindle Apps (Email)

You can import highlights from non-Amazon documents sent to your Kindle app by following the steps below:

  1. Launch the Kindle app on your device
  2. Open the document you want to import
  3. Select the Notebook (top right)
  4. Tap the Share icon (top right)
  5. Select Email
  6. Email to

Readwise has advanced duplicate detection, so you don't need to worry about accidentally duplicating any previously imported highlights.