Letter of Recommendation

So one of my employees actually asked me to write a letter of recommendation for him. He is applying for a Masters in Cybersecurity program at University of California Berkeley. They are a very prestigious college with a rich history and a vast alumni network. I want to make sure he gets the absolute best recommendation that I can give him, to increase his chances of getting into the program. Unfortunately, I have no idea where to start or how to properly write a letter of recommendation. I am looking for someone that has experience in writing recommendations to colleges to help me.