Identify your audience and the particular statement you wish to make in the letter. The audience for most letters of intent consists of the admissions board and professors who teach at the school. Determine the method you will use to convince them to admit you to the program. You can either try to convince them that you are a motivated and gifted student, or that you posses some other traits that you feel will lead to success in the field.
Outline the content of your statement of intent. Include information about your specific interest in the field. If, for example, you are submitting an application to study architecture at the graduate level, specify which aspects or types of architecture interest you. Also include a reason that the particular school you are applying to is a good fit for your desires. Outline any specific experiences or preparation you have undergone that you feel have prepared you to continue your education in this field, such as experiences outside of school, jobs you have held and extracurricular activities, as well as your future goals or ideal career path.
Write the statement using your outline. Avoid using overly stylistic language or technical field jargon. The statement is a persuasive piece of writing, so come across as straightforward and honest as you can in your writing. Back up any general statements with specific and relevant personal experiences, and avoid using catch-all words like "invaluable" and "enjoyable."
Carefully proofread your statement. Have several friends, professors you are close with and any available writing tutors at your undergraduate school proofread your statement. Revise accordingly and repeat this process until you have a draft you are comfortable with.