How do you write a letter of recommendation for yourself?

Be honest. Tell them why you should get the award, but don’t overdo it, and don’t exaggerate. Also admit your faults, but obviously don’t go overboard with those either. Just tell the truth.

When I write recommendations, I try and focus on two key elements of my classroom experience with them that suggest they would be appropriate candidates for the award/internship/whatever for which they are applying. If I mention faults, I always use the strategy recommended for interviews where the fault reveals a strength (e.g., “after failing to secure a grade that she found satisfactory on the first paper—though she still performed better than 90% of the class—[candidate] came to my office hours and asked very insightful questions until she was confident that her next paper would be outstanding”). That’s perhaps a bit much, but I’m sure you get the idea.

Use third person.

Cover the points with data driven results -
Did blah blah blah resulting in yadda yadda yadda percentage increase in productivity.
Use specifics.
Put in x amount of hours resulting in z amount of…whatever.
Always link what you did with results.

Look at the guidelines for the award and use those to fit in your qualifications.

Congratulations on this recognition. I would think your advisor is looking for a list of accomplishments, basically a modified CV in a departmental context, without words of praise loaded on. Let those come from elsewhere.

Don’t embellish but also don’t hide your achievements. It can be very hard to sing our own praises. This isn’t the time to be too modest though. I like @SavoirFaire‘s approach. Keep it honest but don’t undersell yourself.

The same way you would if you were writing it for a friend or colleague.

Make an outline of the high points – things worth mentioning that make the person (you) special. Don’t be ashamed to be honest, but don’t lie, either. Again, pretend you are writing this for a good friend who happens to be you.

Use the same sort of structure you would for any letter:

opening/intro
reason 1
reason 2
reason 3
conclusion

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!