In commercial operations, this is basically dependent on the airline.
Some airlines have Load Control departments/agents that plan and issue loading instructions to the ramp agents (to load freight/mail and baggage in the different holds/loading positions). Passenger figures are derived from the check-in system, with seats per cabin zone easily determined and considered. Close to departure time, the flight is closed (roughly 30 min before ETD) and the system calculates the load sheet, which is then given to the crew (in person or sent via ACARS).
This document has all the flight's load information, actual weights, underload information and also the % MAC CG for ZFW, TO and LDG weights. During the final checks before start, the crew will input the % MAC CG into the appropriate page in the FMC and will get the stab trim setting.
Some other airlines have a more streamlined operation and do not have a Load Control agent to perform all these tasks, so upon flight closure, the crew is given a simplified load sheet with only the passengers and freight/baggage distribution and the crew has to calculate the % MAC CG and stab trim setting. This can be done using a software made by the aircraft's OEM, i.e. Boeing OPT, or if you are brave enough (or it is your lucky day) using a good and old manual load and balance sheet, with the help of tables and graphs. The end result is the same: you get the MAC CG and input it into the FMC.
BF