1. Explain the nature of your professional relationship : That sounds really serious, but it's simply a helpful piece of context that acts as an "introduction" for your recommendation. Whether it's a coworker you've worked closely with for years, a manager, a direct report, a point of contact at an agency, or something else entirely, it sets the stage for the reader to learn why you're writing this recommendation.
2. Provide details of the position for which you're recommending the person : Are you recommending this person for their work in one position? Or are you writing about their work across multiple jobs they've held while you worked with them? Either way, a great next step is to explain some of the notable parts of their job(s). It may feel strange -- kind of like you're listing out their job description. But this is very helpful for anyone reading the recommendation, looking to get a feel for what precisely it is they did in their job. Resist the urge to create a laundry list of their job duties.
3. Indicate how their contribution helped growth : This could be an explanation of how their performance helped hit hard metrics, or it could demonstrate a contribution toward more esoteric things, like leading their teammates or fostering new initiatives.
4. Explain what these achievements reveal about that person : By now, you've included some specifics -- so explain what those specifics mean for the larger theme of your recommendation. Do the examples you've detailed reveal that person is hard-working? Ambitious? Great for team morale? Connect their accomplishments with their attributes.
5. End with a note about the personal aspect of working with him/her : In this section, hit the message home with a mention of how you felt working with the person, your hopes for their career, or simply a prediction about their future.
Before sending it make sure that you proofread. Remember, the recipient has the opportunity to review and request changes to your recommendation, so if you're concerned you haven't written a recommendation in the most helpful way possible, they can still get in touch with edit requests.