It might be worth doing your Nav exam last, even just before the test, which is what I did. Simple reason is that by then you should have a better idea of what's involved due to having practical experience during your solo nav etc.
If I remember rightly most of it was plotting a route on a map, working out headings from winds, fuel and times, then plotting a diversion. The rest was made up of multi choice questions about VOR's etc. If you can master the Wizz wheel then you shouldn't have a problem.
Good luck though,