Make sure to type your statement and do not forget to use a spell-checker and a friend to check clarity and grammar. Enclosing a Vita is generally a good idea. After reviewing programs in Graduate Study in Psychology and Associated Fields and consulting with the faculty, graduate students and friends, choose about twenty or so programs in which you may have some interest. Write to them and request additional descriptive information and application materials. After reviewing all this material plan to actually apply to about a dozen programs.
Many programs reject ten to twenty very good applicants for each one they accept.
You probably need to apply to that many programs to help even the odds. Be sure to apply to the programs that are most desirable to you, but also apply to some programs that appear to be less competitive. A good discussion of the pros and cons of obtaining a Masters degree before the Ph.D. can be found on the Hanover web pages. However, it is a good idea to visit prospective programs and talk with faculty and students there before you are invited. It may help you to decide if you even wish to submit an application.