Letters of recommendation fro a doctor you have shadowed is not really worthwhile unless you are applying to a DO school.
Dear Committee Members,
...the letter starts with a statement that the writer is pleased to write a letter on behalf of the name of the applicant.
Then you say how you first met and when that happened. If there has been a change in the relationship over time, that is described (e.g. you took a class with the writer and then became the writer's TA).
New paragraph, description of what you did.
New paragraph, description of skills you demonstrated or how you grew during this time.
Last paragraph, some statement of how you compare to others, or to what extent the writer thinks that you will be a good doctor (e.g. top 5% of the students who have shadowed me over the past 15 years, or "I would be comfortable having [applicant] care for my own family and home to welcome him as a colleague."
Sincerely,
Name
Title