1) To run a debug flow in FortiGate GUI, go to Network -> Diagnostics and select the Debug Flow tab.
2) By default, the number of packets is 100, maximum is 1000.
3) Enable the filter and there will have two filter types.
4) For filter types 'Basic', it is possible to filter by IP address, Port, and Protocol.
5) For filter types 'Advanced', it is possible to filter filter by Source IP, Source port, Destination IP, Destination port, and Protocol.
6) Once the filter has been configured, select 'Start debug flow' to start the debug. The debug messages are visible in real-time.
7) It is possible to stop the debug flow by selecting 'Stop debug flow' or wait for it run until number of packets that had define.
8) It is possible to save the output in CSV format.
9) The output can be filtered by 'Time', 'Message' and 'Function field'.