Hi You should get your SEVIS registration done before, there's a fee you have to pay to the J1 exchange site mentioned on the Induction Ds-2019 form. After that you will have to fill form DS-160 to get an appointment and request expedited one by attaching your letter. If you have any issues or concern try to get in touch with the legal department of Mayo Clinic (researchvisa@mayo.edu) Hope this was helpful. I also start around that time so good luck and see you then Marketa 🙂