What did you do? How did you do it (particularly what was unique about how you did it)? What were the results? What impressed you about the work you did (you, in this case, being your advisor). Why should the committee give you this award (summarizing the unique and innovative techniques you used to get results). Why is this work important? Put it in context. You also might put the contextual information first. Why did you do this work? Where does it come from?
What your advisor wants is the broad outlines of who what where when why and how. It’s what we do when we do favors for people. We ask you to do the work so we don’t have to think it all up, Then we take it, and we edit it (unless we don’t care too much or it is already perfect for our purposes).
Usually, when you write your own recommendation, you come up with things that I, if I were to write it, wouldn’t think of. You also create a structure for the document. Hopefully, all I have to do is make it sound like me. I also might add one or two ideas I have that you didn’t have.
But I don’t think it’s something to spend a lot of time on. Get down the broad outlines and some details and then let him whip it into shape. You’re saving him work. And you get a good deal out of doing that for him.
And congratulations, by the way!