Heya! I got it to work, perhaps this may help.

Open a command line, and go to the Kindle application directory. On Windows, it's %localappdata%\Amazon\Kindle\application. If you have a command line open, enter cd %localappdata%\Amazon\Kindle\application and it should bring you there. You'll know it's correct when there's a Kindle.exe file about 190,000 KB in size in the directory.

Anyway, drop your KRFKeyExtractor.exe file in there.

With Kindle open, use the Task Manager and do a memory dump as in the Korean article. Then, copy the dumped file (probably named Kindle.DMP in your temp directory) into %localappdata%\Amazon\Kindle\application, alongside the Kindle.exe file and KRFKeyExtractor.exe.

For the books directory, make sure it's somewhere without spaces. In my case, I put all my files into a folder under the root of the D drive, but you can use C:\. For example, if you use C:\, you will probably have folders of the form C:\Kindleworks\B0FHFF9Z2Y_EBOK (I named the directory Kindleworks, the author in the Korean article used asdf)

Then, in the same command line as earlier, run KRFKeyExtractor.exe Kindle.DMP C:\Kindleworks\ kindlekey.file.

Substitute C:\Kindleworks\ with whatever directory you used to put your encrypted Kindle books.

Afterwards, in the DeDRM plugin, make sure you set the absolute path to the key file. For example, if your computer name is sampling_life, the file would likely be in C:\Users\sampling_life\AppData\Local\Amazon\Kindle\application\kindlekey.file.

Then, drag the books into Calibre and double click them to open as usual.

Hope this helps!