Once you create your route, you can then add Driver sessions by creating a new session. (Suggest do not use the 'Default' session automatically created).
The procedure is relatively simple, but can be very labour intensive and take many months if you are creating complex sessions involving scores of consists scurrying about your route delivering goods, services and passengers to multiple interactive industries, ports, airports and stations.
For a simple route, 1st select the route in Surveyor mode and then choose 'View Sessions', then 'Create Session'. (To edit an existing session, select Edit Session; To Drive it - select Drive Session, etc).
During Session Creation mode, add your locomotive, consist and driver to the track at a suitable start-point. Then go to the 'Edit Menu' (top left) and then choose Edit Session... and give the driver some commands to carry out in the Driver setup dialog (Select Driver setup, then click on the Edit command, bottom left).