There is no such a single counselling session which you have to attend. There are mainly 5 rounds of counselling after each cutoff released by the University. After filling the form,  you have to wait for the first cutoff to come out. After each cutoff,  University provides students a period of three days to report to the college they are interested in,  given that they are clearing the cutoff. You just need to do this to get admission in DU.