Admissions or the registrar’s office is generally the department you need to seek as they are the section that registers you for classes and keeps track of your academic career. They should have a form for you to fill out to get a record of your certifications.
You will want to formulate the letter with your name, address, and contact information on the left hand side. Underneath this area the school name, address, and contact information should be placed. Your information and the school’s can be single spaced, though a double space is needed between you and the school’s information. Double space again and either use dear or other address. Preferably you will have a name to send this letter to.
The next part is the body of the letter. It should be formulated in a business tone such as ’I attended your college (name) between (dates) and earned (list the certifications). I would like to request that each of these certifications be mailed to the following address.’
If you are looking for the college to provide written proof that you have attended the school such as a certification request letter the content will be slightly different. In this case you are asking for the college to state when you attended the school, what you studied, and what other organizations you might have been in.
Most colleges will have a form for you to fill out, so you could ask for this form to be sent to you in order to get the documentation you need.