How do I write an essay comparing two different pieces of music?

Firstly try to find any similiarities that there are - do they have the same instrumentation? Or do they both come from a similar style - are they both dances, for example? Discuss these first, and don't forget to state the obvious - even though it might seem pointless to say that they both have big flute solos because it seems to obvious, still write it down - you can still get marks for it! After you've finished the similarities, do the same for the differences, and again state the obvious - even if one is for piano and one is for symphony orchestra, you should still include this in your answer as you can write more about it beyond that as well - how does the different instrumentation affect the feel or mood of the piece? 

After you've finished writing about similarities and differences, try to draw some kind of conclusion from them. If the question asks you about tonality and harmony, for example, you could talk about how despite there being some similarities, there are differences between the two pieces, and these show the development of harmony and tonality through the western classical tradition (or whatever tradition the two pieces come from).