i did this assignment aswell and you really need a lot of analysis of sources and a strong argument with a clear thesis.
i would start by just reading about the whole topic and then picking just one aspect of the fall of the dynasty and this will allw you to analyse sources about thhis aspect...
reliablility = whether what the source is reliable in what its saying eg. is a german source from 1940 going to be reliable in its evalutation of Britain etc.
usefullness = even if the source isnt reliable in its information, is it useful in the way that it portrays certain views or the bias...