Any letter should be concise, clear, and appropriately polite, as well as showing knowledge of the person or company’s needs or requirements.
Start the letter with your contact address (postal and email) on the right-hand side, followed by the date on which you are writing the letter (which should be written out in full, for example, ‘May 15th, 2011’ rather than ‘15/05/11’).
Try to find out the name of the person you are writing to if possible: Using their name rather than writing ‘Dear Sir/Madam’ or ‘To whom this may concern’ shows that you really care about the cause to which you want to donate your time or service.
Your letter’s first line should indicate why you are writing to the person or company; say precisely what service you are offering to donate and why you would like to offer it. This would also be a good time to mention how you heard about the person or company, how you know that they require this service, and what you personally can offer by donating this service.
You can then use the rest of the letter to expand upon your credentials (why are you qualified to offer this service?) and experience (if you have donated a similar service before, you can talk about what you achieved as a result of this and what the person or company you donated the service to was able to gain from it). If appropriate, you can also include your availability, and other contact details such as a telephone number.
Finally, before signing off with "Yours sincerely” (if you don’t know the person’s name) or "Yours faithfully” (if you do), you can include a sign-off line that sounds co-operative, reassuring, professional, and confident, such as "Please do not hesitate to contact me should any further information be required” or "Kind regards”.