I've been searching for a set of guidelines I can give to my dentist/professors to write my letter, but I haven't been able to find much. Does anyone have a link or document that I could give to them?
Honestly though, your letter writers should already have a good idea of what/how to write your letters. I suggest providing them with information about yourself, your resume, transcripts, your PS (or just a draft) or even a summary of your life events and and goals/plans. These are things a letter writer can utilize in writing your letter if they don't already know you well enough.