This statement is very similar to the personal statement that you would write as part of a graduate school application. This is where the reviewers learn all about YOU, the person; you only have three (3) pages to paint the picture.
Your objective is not to regurgitate the information on your resume. This does nothing to describe you, the person. However, if you treat the statement like a diary, you run the risk of being too personal or informal. Since this is a personal statement, you should drop the scholarly tone that is typical of technical reports. Most importantly, there are subtle ways to highlight your accomplishments without appearing self-centered.
Your objective is, however, to gratify the reviewers and hold their attention. This is most easily accomplished by becoming a storywriter. Honestly, just be yourself and tell your story while addressing what is asked of you in the writing prompt. If you hit a wall, try speaking aloud; this will likely evoke details that you forgot all about. Light humor is fine, but don't feel like you have to make your reviewers burst into laughter.
Avoid glorified statements like, "from the day I was born, I knew I would become an engineer." But if you do make a claim like this, you'd better have a compelling story to support it. Ditch the "big words" and utilize smaller ones (i.e. use vs. utilize); you want this statement to be easily read. Practice using active language to make your meaning clear for the audience and prevent your sentences from becoming too complicated.
The title of this statement is long for a reason. You cannot neglect relevant background and future goals. Relevant background is most often previous research and teaching experiences. Your future goals should explain what you want to do, but more importantly, why? Don't just list what you did. Intellectual Merit and Broader Impact is all about illustrating what you learned, how you will apply it in the future, and who you will influence. Even though the statement should address the criteria throughout, it is in your best interest to summarize them in a separate section at the end.
My sample statements and reviewer feedback are provided in links and references.