For booking your flight, you have to be registered at FSAirlines and joined to Brown Pelicans Virtual Airlines.
If you did so, you can book your first flight. It does not matter if it is a training flight or a regular scheduled flight, the procedure is the same.
However, this guide demonstrates the procedure for Trainee.
Booking the Flight
After login in and clicking Crew Center link, find Personal menu on the left side of Crew Center page. Click Book Flight.
You'll get to Book Flight page. As Trainee you will see only one flight for booking: MBPV - MBSC. It's the first one of your two training flights.
At the end of the record, click the check icon.
Now, you'll be prompted to select an aircraft for the flight. There is only one option in this case, so select C172 and click Continue button.
The flight is booked now, just click continue.
The Briefing Document will be open. You'll find some information over there such us: Distance; Estimated Fuel; Number of Passengers; Weight of Passengers, Baggage and Cargo; etc.
Note especially the information on Estimated fuel: 33lbs. This number + reserve should be tanked into the aircraft.
The flight takes about 35 min and the nearest alternate aerodomes (MBGT, MBAC, MBPV) are in the range of next 35 minutes of flight. So if you tank 66lbs of fuel, it should be quite enough for the flight and reserve.
Now it's time to open your simulator and connect and setup FSAirline Client.