There should be a save button at the bottom of the last page of the form. You can work on it, save it and then come back to it later. However, once you submit it, it's gone from your computer.
A word of caution for someone who just finished their first bpo for reotrans-budget enough time to finish it. It took me longer than the CC or LandAmerica form.
And, no, there are no instructions.
Good luck!