Download the latest tools package, and unzip it. (On Windows, right-click and “Extract All…”; After extracting all, rename the tools_vX.X.X.zip file to tools_zipped_vX.X.X.zip to prevent later confusion)
Run calibre. From the Preferences menu select “Change calibre behavior”. (Do not click “Get plugins to enhance calibre”, that option is reserved for ‘official’ calibre plugins.)
Click on Plugins (under “Advanced”) — it looks like a jigsaw puzzle piece.
Click on the large “Load plugin from file” button
Navigate to the tools folder unzipped in step 2
Open the “DeDRM_calibre_plugin” folder
Select the DeDRM_plugin.zip file in that folder
Click on the “Add” (sometimes “Open”) button.
Click on the “Yes” button in the “Are you sure?” warning dialog that appears. A “Success” dialog will appear, saying that the plug-in has been installed. Click on “OK”.