What you should do is to prepare the following to give to the professor:
- resume
- dental applicant essay (rough draft is okay)
- transcript & class test/assignments that you had with the professor
- if you have time, write a short essay that includes everything that has not been said from above
- also, when you meet with her, give a little information about why you want to enter dentistry and what activities have you done to get closer in obtaining this goal.
A good dental LOR in my opinion includes your academic achievements, your personality and characteristics, your contribution to your university and/or community, your capacity to handle dental school in the opinion of the professor.
However, because there are no restrictions on what to right on the LOR, just give her the documents from above and based on these information, as well as the prof's impression of you from the time you had her class, she will be able to write a very detailed LOR.