Your best bet is to tear into an old junk tranny, maybe an old 3spd, and see if you can get it back together.
Most trannies are similar, so once you get familiar with them you won't be lost, but I'd hesitate to tear into a 5spd for the learning experiance.
Other than that, search out some transmission sites, specifically some exploded views of the tranny and realy look at it.
Most problems with manuals stem from the synchros and loose bearings, input shaft and output bearings can 'normally' be swapped out without tearing into the trans itself too deeply But that depends on the tranny design. (same goes for the needle bearings between the input and output shafts, but don't drop them inside!) synchros need the tranny torn down as you have to remove the gears, and you may as well change the worn bearings while you're in there.
it does take some experience, but if you have patience and are able to take some photos you can do it.
Most often the countershaft bearings and gears will be fine, most wear is confined to the input and output, but if you open it up and see a lot of pitted, badly worn or galled gears, I'd look for a new tranny, gears get expensive and you could easily spend as much on a few new gears as you would on a new tranny.
ken.