Plan your writing the same way as for an oral presentation.
Step 1 - Analyse the audience – to whom am I writing?
Step 2 - Determine the focus and purpose – why am I writing?
Step 3 - Prepare the topic – research or read about the topic.
Step 4 - Collect and organise the information – introduction, body, conclusion.
Step 5 - Write – letter, memo, e-mail, standard operating procedure.
Step 6 - Ruthlessly re-write.
Step 7 - Proof-read and proof-read again.