the usual format for a professional letter is fine. One inch margins, letterhead, etc. As best as I recall, the letter can be printed, signed, scanned and uploaded as a pdf into the AMCAS or Interfolio website.
Here's the formula that most professional people follow in writing a LOR:
Dear Admissions Committee (or To Whom It May Concern: )
First paragraph: I am writing in very strong support of the application of [first name last name, AMCAS number] to medical school. I have known [first name] for __ years. The first paragraph goes on to describe how you know each other and the evolution of the relationship (e.g. the pastor met you when you joined the congregation 4 years ago and got to know you through your engagement as an assistant at the Sunday School and a volunteer at the annual Ice Cream Social).
Second paragraph: Skills you have demonstrated (see AMCAS brochure for the information) with examples of activities you've engaged in that demonstrated these skills. Ideally, these are things the writer has witnessed, not a regurgitation of the experience section of your application.
Third: Softer skills, demeanor, personality. Again with examples of how you have demonstrated these attributes. (e.g. a little story of how you reacted with a cool head when you were the only adult with 40 children back stage at the Christmas pageant when the fire alarm went off)
Fourth paragraph: Something about how you compare to others the writer has known who went on to medical school (or professional school in general). Sometimes the writer will classify the applicant as being in the top X% of a cohort or one of the best in my x years as a [pastor, instructor, clinician] at [name of the place(s) where one has worked]. Some will say something to the effect that they hope someday that you will be the doctor for their family or that they had hoped that you would enter their profession (laboratory, etc) but that you have clearly demonstrated your keen interest and aptitude for a career in medicine and they are confident of your success in the field or how fortunate patients will be to have you as their physician.
Sometimes a letter writer will offer a telephone number and an offer to answer any further questions if needed.
Closing (Sincerely, or whatever the writer generally uses)
Signature
Printed name and title.