I’ve personally found this method to work (Windows + Kindle desktop app) in 2016:
1. Install all the software you require (Kindle Windows app, Calibre, DeDRM plugin, Python, and pycrypto)
2. Open the Kindle app for windows and login. let it download a copy of your book to the computer. This book will be in the “My Kindle Content” folder that you’ll find in your Documents folder.
3. Open Calibre and then install the DeDRM Plugin from Preferences>Plugins>Load plugin from file
4. After it’s installed, it will be highlighted. Click on “Customize plugin” and a Configuration dialogue box will open.
5. Select “Kindle for Mac/PC ebooks” and another dialogue box will open. It will most likely be blank (no keys shown.)
6. Click the green + button to create a new key. a dialogue box will open with an already suggested “default_key” in the input field. click OK.
It will be added to the list of keys and it will be the only one you see. Click Close and then OK to exit both dialogues.
7. Now we have a key added to Calibre. try opening Calibre with debug mode (so you can see if DeDRM worked) by opening Command Prompt and typing “calibre-debug -g”. Calibre will open normally, but command prompt will remain running too.
8. Locate the book you downloaded (“My Kindle Content” folder) Drag and drop it to calibre. Once it is imported, go to command prompt and you should see something like
“Decrypting. Please wait . . . . done
Decryption succeeded after 0.1 seconds”
Then as a proof of success, your ebook should open up when you double-click on it in Calbre!