Write what you would've written without a word limit. Read it. Scrap everything that's superfluous or not absolutely necessary. Try to put the sentences that are necessary even shorter, for example by changing the word order. Reread it to see if it is readable.
Instead of digressing on a point you could write short, semi-rhetorical questions and let the reader think about them. Thus you'll be able to communicate a lot with just a few words. Try to avoid metaphors, long prologues and the like.