Well, the letter isn't too stressful. Mine, I just wrote a bit about myself, such as my university and my major. Then I mentioned my two CPs and what my jobs on them were, then I wrote a paragraph about why I thought I'd be a good fit for the rep position.
The interview is pretty similar to the CP one, it's done over the phone with your recruiter over your school. Nothing stressful. I got in within about a week of the interview (on my birthday, actually, best gift ever!) and you don't have to do a formal acceptance. Your recruiter will just send you an email.