when the petition leaves the NVC and it arrives at the embassy, the embassy will send a a packet of information (or just a single letter, it depends on the embassy) to the beneficiary. This is typically called packet 3. packet 3 contains information on the next steps of the visa process. usually including what needs to be brought to the interview, how and when to schedule the medical exam, and how and when to schedule the visa interview. and it will sometimes include things that you need to send to the embassy. for us once we got packet 3, we had to fill out his DS-160 and email the confirmation to the embassy. once we did that they emailed him the link to schedule his visa interview.
the medical exam should be done at least 2 weeks before the interview, this gives the civil surgeon enough time to send the results of the exam to the embassy. and the soonest my husband (then fiance) was able to get a medical exam wasn't until middle of June. so we scheduled his interview for july, to give the doctor enough time.