"The actual Enumeration shall be made… in such Manner as they shall by Law direct."*By Rabbi Jack Abramowitz
On the first day of the month of Iyar in the second year of the Exodus, G-d spoke to Moshe in the Sinai wilderness and told him to take a census of the Jews, counting males from the age of 20. Moshe was to be assisted by the Tribal leaders, who are then named. (The most famous among them is Nachshon ben Aminadav of the Tribe of Judah.) Moshe and Aharon gathered the designated leaders and they reviewed everybody’s documents, taking the census as they had been instructed.
*(This synopsis title isn’t from the Torah; it’s from the US Constitution.)