On the application, you just input the name and email address of the recommender (and maybe some other info, I can't remember), and the system automatically sends them a link with instructions on how to upload the letter.
As for the types, I had 3 science professors and 1 dentist. It depends on the schools you're applying to, and you can only upload a maximum of 4 letters to the application. I think you should keep it as is, with 3 professors and 1 dentist. Sometimes school will accept additional letters that you mail directly to them, but you should ask them on how to go about doing this. Hope this helps