Filing a flight plan for a VFR flight is not required by regulations; however, it is a good operating practice, since the information contained in the flight plan can be used in search and rescue in the event of an emergency. Flight plans can be filed in the air by radio, but it is best to file a flight plan by phone just before departing. After takeoff, contact the AFSS by radio and give them the takeoff time so the flight plan can be activated.
AIM 5-1 Preflight
PHAK Ch 16 (21) Filing a VFR Flight Plan
91.153 VFR flight plan: Information required
FOM Ch. 8 Cross-Country Operations
Private Pilot ACS 6 Task D. Cross-Country Flight Planning