Simplest explanation (no details, no potential complications included) on how to get on protected mode:
Set the lowest bit of MSW/Cr0.
Simplest explanation (no details, no potential complications included) on how to get off protected mode:
286 - reset the processor (on PC-compatibles, use the keyboard handler's processor-only reset or triple-fault the computer).
386+ - clear the the lowest bit of the MSW/Cr0 "Information has a tendency to be free. Which means someone will always tell you something you don't want to know."