technically you can fill out and submit the DS-160 whenever, it is valid for 1 year after submitting. just make sure you print out the confirmation page as you will need to bring it o the interview and (possibly) the medical exam.
the letter that the beneficiary should be getting from the embassy (also called packet 3) should tell you how to do the DS-160, how and when to schedule the medical exam, how and when to schedule the interview, and what documents need to be brought to the interview.
you could also try emailing the consulate to try and get that information.
this is the email they have listed
Canada.Visas@gdit-gss.com