*suck up, suck up* "would you write me a letter of rec for med school" *suck up some more* "Med schools require the professor to cosign, is that ok?" *suck up and thank profusely*
if this person has ever written a rec letter before they probably already know a prof will have to cosign. If they haven't, I doubt they'd be offended if you told them it's basically a requirement.
Don't make **** up, we know that you don't
need a professor to cosign it, only that it looks far better if they do. We also understand that a letter only from us doesn't carry all that much weight.
As a grad student that's written a bunch of letters for former students of mine, here are my suggestions: first, make sure that the TA knows you well enough to be able to speak to your abilities as well as your personality. Second, offer to give them more information about yourself. I usually ask for a resume, and it's even better if I get a personal statement. Usually I make the student sit down with me to discuss career plans, why they want to do those things, etc. Finally, make sure they're going to write you a
strong letter, and ask if they've written letters for students before. It takes practice to be able to write a good letter of recommendation. I'd also suggest not hounding them, and give them an adequate amount of time to write a strong letter. You don't want your letter to be something that they rushed to get done.
You
do not want a letter that only talks about your performance in a class or your grade in that class. That's worthless. You want a letter that can sell you as a person, including how you interact with other people, how you contribute to discussions, what your academic abilities are like, what your real strengths are, etc. Give your TA something to work with.