If you have a CV or resume I would give that to your professor. Maybe you can write a little personal statement also. Let them know that you can also sit down with them and discuss anything they have questions.
Let them know the exact name you used to registered for AMCAS as well as your AAMC ID#
First and foremost, thank them for agreeing to write you a strong letter by a specified date — letters will likely roll in late if you rush the writer without prepping them properly or allow the responsibility to fall onto them to ensure timely completion.
Give a very brief narrative about you and your intentions (a short paragraph).
Stats: give them a summary of your GPA/CV, favorable or not. For their peace of mind include a PDF attachment of your transcripts/CV, let them know it’s attached and the correct title if you have multiple attachments. Highlight the pros of your stats, offer to talk more in depth in person about personal circumstances that may of left bruises on your transcripts.
Concretely tell your writer what types of things you’d like them to address.*
Give them a concrete way to submit the application. This will mean giving them the physical address of the AMCAS letters service if they want to go snail-mail, or providing them the links and steps to upload your letters online. If your letter writer finishes the letter and can not send it because of poor instructions it’s not the fault of the writer.
Don’t be surprised or insulted if the professor who agrees to write you a strong letter first requests for you to draft a letter of recommendation for yourself then submit it to them to modify as template (it’s very common in the research world). Above all else you should always know your strengths and weaknesses of your application.
Go to Interfolio.com and make an account there. It's like $15 for 6 months or something. And follow directions on the site, about how your letter writers can upload confidential letters to the site. Which you will later upload to your AMCAS application system.
Does your school have letter services or pre-med office that compiles and sends letters to AMCAS for you? If so, you can send these letters to them and let them handle.
If not, check out Interfolio and compile your letters by yourself. You can send these letters to AMCAS directly without using Interfolio (I could be wrong on this), but usually people use either Interfolio or pre-med committee/pre-med letter service within college.
To be clear, you never actually physically or digitally receive the letter, it's written and sent straight to AMCAS. You have to waive your right to read it by clicking a box when you apply, all most all schools require this.