The last I looked into it, the best way to do it was to get an older kindle so you could download the older DRM copies of books from amazon. But I think some newer books are using only the newer DRM which I don’t think has been cracked.
It has probably been at least a year since I checked. If you do end up finding an updated method, I would be interested.
That’s fine and easy on desktop/web browser, but for mobile devices it is not quite as easy. You would either need to use a hacked version of the app or a third party app.