Martin Parkinson has published some useful advice on the preparation of block diagrams here.
For students on m883 some of the general advice is relevant, so here are a couple of key points:
If you are asked to draw a diagram, it should be your own. "Your own" means not just one lifted from the textbook or a website. You have to draw it yourself - but you can of course base it on a diagram from the text or a website. If your diagram is pretty much a copy of one found elsewhere, then underneath it you should write "Figure adapted from [URL, or book ref]"
You really don't have to produce a computer drawing - in fact, unless you are a whizz with a graphics package, you're better off drawing on old-fashioned paper, and taking a photo which you then insert into your TMA. For most people this is much faster.
I would add that, if the question refers to an example in the set book or study guides, you should make sure that your diagram contains all the necessary elements. Remember that these are formal modelling artefacts and should be complete and consistent in order to be useful.