Write an article based on this "Keep it professional. Format your letter. Begin your letter. Write the first paragraph. Write the second paragraph. Write the third paragraph. Write a professional closing. Sign the letter."
You should write your letter the same way you would if you were applying for a paid position. Be professional and consistent in your letter, as a poorly-written letter of interest could cost you the position.  Use a uniform type size and font that are appropriate for a business letter. Stick with a font size between 10 and 12 points, and choose a font that is easy to read and has clean lines, like  those in the sans serif family. Some examples of professional-looking fonts include Arial, Century Gothic, Futura, Lucida Sans, News Gothic, Technical, Times New Roman, and Rockwell.  Do not use any unusual or unique colors in your letter. Type the body of your letter in black. You'll want to follow proper formatting protocol to make your letter look as professional as possible.  Write your contact information in the upper lefthand corner of the document. Include your full name, address, telephone number, and email address.  Insert two or three line breaks, then write out the contact person's information. Start with his or her full name (or a proper title, if no first name is given - for example, Ms. Stone), the department that person works in, the name of the organization you are hoping to work with, and the organization's address.  You may wish to include the date on your letter so that the person reviewing your application can see when you applied. This can be inserted between your contact information and the organization's. The first thing you should do (after including your contact information and the organization's contact information) is to address the letter's recipient with a proper title. If the recipient is a doctor or has earned a doctorate, address him or her as Doctor. If not, use Mr. or Ms. If the gender of the recipient is unclear, use the person's full name rather than a title. If there is no contact person listed in the announcement and you cannot find out whom you should address your letter to, you may be able to get away with using a subject line instead of a formal salutation. This part of the letter is the venue in which you will market yourself to the administration at your organization of choice. You want to introduce yourself and make your intentions known right from the start.  Include a statement of interest in the position. Mention where you saw the announcement for this volunteer opportunity. Write two to three sentences outlining your experience and expertise in the field of your interest. If you have a formal education or have taken coursework in the field to which you're applying, mention that in this section. Reference the organization's mission statement or values, and address why that mission or set of values is important to you. You should also tie your education, experience, and/or interests to the company's values or mission statement. Let the hiring party know that you are willing and able to work toward the organization's goals. Now that you've introduced yourself and formally declared your interest in volunteering your time and efforts, let the hiring party know a bit more about yourself.  Discuss your employment and volunteer history, and talk about how that experience it is relevant to the position. If your previous jobs are not directly relevant to the volunteer position, then mention your employment history as a way to highlight your specific strengths as an employee. You may want to focus on your work ethic, your dedication to your current or former employers, and any transferrable skills that could be relevant to the volunteer position you're applying for. Name any skills you possess that would be useful or relevant for this position, and talk about why those skills would be useful at the volunteer organization. If you have any substantial achievements that you are proud of (and that have relevant or transferrable skills), detail how those achievements make you an ideal candidate for the volunteer position you're interested in. Include any major issues or problems you were successfully able to identify and solve at your current or former jobs or internships.  Discuss any ways in which you successfully innovated new policies or procedures at your current or previous jobs or internships.  Incorporate any instances that illustrate your ability to take initiative and show responsibility or leadership skills. The first two paragraphs should have successfully introduced you as a volunteer candidate, discussed why you're interested in the position, and illustrated why you are a viable candidate for the position. In your closing paragraph, you'll want to wrap up the letter by laying out what you are able to commit to.  Let the hiring party know how much time you can commit to each week. If you have specific days or times that work better for your schedule, let the hiring party know when you're available to volunteer, if you are accepted into the organization. Do not assume that you have the position. Writing about yourself and your availability as though you already have the position could be a turn off for the organization's hiring party. Ask the hiring party for the opportunity to meet in person to discuss the position further, and let him or her know when you would be available for an interview. It's important to be flexible when trying to set up a meeting/interview, so try to keep your schedule open and be willing to come in on short notice. You'll want to formally thank the hiring party for his or her time and consideration. Use formal, business-appropriate language, including a professional sign-off, such as "Sincerely," "Warm regards," or "Best regards." Provide both a typed and handwritten signature. If you are emailing the letter to the hiring party, you may need to print out a copy of the letter, sign it with a pen (use black ink to match the type font), and then scan the signed letter into a PDF file.