Tips on how to write to an allotted word length

Know how much space your handwriting takes up on a page so you can work out how many pages to write.
If you're writing on computer, how many words fit a page?
Using size 12 font it's about 500 - use word count to double-check!
Consider your audience - choose form, tone and vocabulary to suit.
List the main points you want to make.
Sort them into order.
Eliminate peripheral points.
Decide how much space you should give to each point.
Balance the spaces you allot for your introduction, development and your resolution - and don't run out of space for your conclusion.
If you have 600 words, your intro should be roughly 150 words, the body of your piece 300 words and the conclusion 150 words: balance!
Know where you're going.
Do a plan/plot outline and stick to it don't get side-tracked.
Have one idea per paragraph.
Don't waffle on to fill the space - it'll probably be rubbish.
Do a rough copy and leave it for a while, preferably overnight.
Be objective when you self edit - pretend someone else wrote it.
Be critical, use a red pen - It's an amazingly satisfying process.
Read and re-read to check that you're writing makes sense and communicates what you intended to say.
Read it out loud to more easily pick up mistakes or contradictions.
Delete words, phrases and sentences, even whole paragraphs, that don't advance your story or argument.
Good self-editing should be honest and ruthless - then your piece will be tight and impressive.