The only training you really need is to cheat a little at the beginning.
The ctrl+leftclick method has proven valuable but at my installation I could still use the 'plot course' icon, which wouldn't let pe actually plot my course but gave away my exact current position.
This and the theoretical knowledge should give you enough opportunity to practice. You can use the cheating methods to compare your results to your exact position and so on. So you can learn while playing the normal career mode
. That is more fun and saves load times anyways.
Be warned though that sometimes there are some differences between the position you calculated and your actual one, no matter how careful you are. Sometimes it seems as if the time of sunrise isn't really acurate. The sunset times even don't seem to work at all.
Read this thread or:
http://www.subsim.com/radioroom/showthread.php?t=116837
this one for some experiences we made so far with the mod.
The manual that came with it explains the use of the sextant fairly well.
Have fun, i.