Would apply 5 months in advance but you can technically apply now if you want. You may want to apply early because there is a chance you could get refused. This would allow for time to apply again. You will have to include updated admission letter for the Sept 2020 start date. No need to apply when in Canada but it often works out this way because most people have much longer processing times than
@MsCutler who seems to have extra fast processing. Including SOWP with study permit often slows down applications and many want to secure WP while not wanting to seem that they are only seeking immigration. Assume
@MsCutler is pursuing masters or PhD.