Finally, I've found somebody who spells out what all of these mysterious (to me) Windows "toolkits" and "upgraders" etc.. are actually doing. Here's the step-by-step guide for doing it in a simple, driver-free way, from the command line (on Linux, but should also work from OS/X or WinXX):

http://www.cvr.cc/jelly-bean-on-your-galaxy-nexus-phone/