I refined my GED/timed writing essay practice book into a small guide/checklist to use during timed essay writing practice. The kids are required to follow it and turn it in with their essay.
On a related note, I came across this site with GED style instructions for writing, a writing plan (which I used in the companion above) as well as 5 sample writing prompts.
http://www.unm.edu/~tinan/writing/ged_essay_directions.htm