H1B back to China

Dear Xiao Baitu and other people who have done this before,

I am planning to go back to China this summer. First time here. I have a few questions:

- It seems to get a return visa, I will have to go to Canadian US embassy. Do they issue visas the same day?
- I got the phone number, have not called them yet. Is it difficult to arrange an appointment?
- Read that I will need a letter from Employer, what content should it be? Just to say I am employed there?
- Any other things I should get before going to them?

This is my first time to go back, sincerely need some help!
Thank you very much for any reply!