Hi- I have what I've found to be a really useful and simple article on this, but I've just tried to 'refind' it online and I cant for the life of me remember how I accessed it. The reference is Knopf, J; Doing a Literature Review, PS: Political Science and Politics, 2006, 39
I definitely accessed it online, so might have been through uni library or athens; hope you have more luck than I have had and that you find it useful if you do manage to get a copy...