Hi
I did some practice ones using graph paper as well as using the computer. Its much easier doing it that way and then you can visualise it in your mind. Not sure what books you are using but I use the Jeremy Pratt books which shows you different ways of working it out. I find the Pratt books really good this way as he always gives you that little bit extra knowledge for each section of navigation.
Good luck with your nav exam - got mine coming up soon! Have you done your qualifying cross-country yet?