If you school doesn’t use a “brag sheet” you could find one to use on the web anyway. This gives your teachers ideas on what to write about. Our HS brag sheet includes activities that you participated in, personal goals, etc.
By you providing this information, it might appear that your teachers know you better than they do.
I also agree ask early while teachers are not over-loaded. Be sure to communicate (via email or in writing) your deadlines well in advance.