Grab the reader's attention with an interesting first sentence for your statement of purpose. You might start out by saying something unique about yourself that relates to engineering, or state a fact about the engineering field that your audience will find interesting and then explain how that fact relates to your decision to enter the field.
Create the thesis for your statement of purpose. This is the backbone of your writing. It is a one-sentence explanation of the purpose of what they are reading. For a statement of purpose, your thesis statement should explain what has attracted you to the field of engineering and what drives your passion for the field. Your thesis statement belongs in your first paragraph.
Explain to your audience in at least one of your body paragraphs why you are academically prepared to enter an undergraduate program in engineering. This is a challenging path, and you need to prove that you've had enough experience and have gained enough knowledge that you know this is the path you want to take and that you will be successful.
Describe what your ultimate professional goals are in at least one additional paragraph. Talk about what you plan to do after you receive your undergraduate engineering degree, such as attending a graduate level program, and describe what you hope to accomplish with your career.
Relate your experience and career goals to the specific undergraduate engineering program that you're applying to. Include elements of the program that you find most appealing and why this particular program is the best choice for what you hope to accomplish with your engineering degree.
Show your audience the information you include, rather than simply stating it. For example, if you state that you've had lots of experience in the field already, show that this statement is true by talking about internships you've completed, research you've done or related courses you've excelled in.
Sum up the main points you covered in the body of your statement of purpose in the final paragraph. Then, reiterate what you plan to bring to the undergraduate program should you be accepted. This is basically a highlight of your best attributes.