To help you compose an effective recommendation letter that you've agreed to write, first look at the job description to which the person is applying. What are the qualifications and skills needed? Try to group those requirements into two or three main categories. The requirement states, for example, that the applicant must have strong communication skills to interact with clients and various levels of management on a daily basis. And, a strong work ethic because the applicant will need to be self-motivated; working with limited direct supervision.
Now, think about the person. And do you have evidence and support that he does qualify for the position? If so, begin with a general statement that includes his basic overview. Here's an example. Jon Sanders' communication strengths, people skills, and work ethic would make him a valuable addition to your company as a sales manager. I highly recommend him. Using words or terms that were in the job description will immediately get the readers attention since you've connected those requirements with what the applicant has to offer.
Additionally, you have identified the strength of your recommendation with highly recommend, or just recommend, if you feel more comfortable with that. You want that opening paragraph to send a message to the reader that Jon is a worthy applicant, so she will keep reading for specific support.