The short answer is: use mobidedrm.py.
This is a python script that must be run from the command line. It can be difficult to learn the "command line", but the good news is that the
Kindle Visual Guide, once decimated by Amazon's strong arm tactics, again has sections on Command Prompt Vista/XP/Mac and Install Python Windows.
There are various threads about mobidedrm, but no explicit set of instructions for its use. You might find what you are looking for in one of those threads, see
search for mobidedrm.