The maths, at the end of the day is really the easy part, and with the use of sight reduction tables, it gets easier, and apparently, using a calculator makes it easier still. One of the links Thomas posted leads to some good stuff on celestial nav, with an easy to follow explanation of how celestial nav works. You dont need to know this, but having an insight to the principals is good.
Where most leaners go wrong is in the actual taking of the sight, not getting the sun, moon, planet or star down to the horizon properly, that just needs practise, and it helps if you know someone who is experienced in taking sights.
I was lucky, when I first went to sea, sextant was the primary means of fixing when deep sea, so it was normal to take half a dozen sun sights, plus full sets of stars at each twilight on a daily basis