Flight plans can be filed through Leidos at 800-WX-BRIEF or with most EFBs like ForeFlight. They can be filed in the air as well as Jason Miller describes.
a. Flight plans which specify VFR operation for one portion of a flight, and IFR for another portion, will be accepted by the FSS at the point of departure. If VFR flight is conducted for the first portion of the flight, pilots should report their departure time to the FSS with whom the VFR/IFR flight plan was filed; and, subsequently, close the VFR portion and request ATC clearance from the FSS nearest the point at which change from VFR to IFR is proposed. Regardless of the type facility you are communicating with (FSS, center, or tower), it is the pilot’s responsibility to request that facility to “CLOSE VFR FLIGHT PLAN.” The pilot must remain in VFR weather conditions until operating in accordance with the IFR clearance.
You can also file a flight plan that starts with IFR and then becomes VFR, but I don’t see the point. You can cancel anytime or just ask for VFR-On-Top if you are in VFR conditions and not in Class A (or possibly Class B or C depending on circumstances).
A pilot departing VFR, either intending to or needing to obtain an IFR clearance en route, must be aware of the position of the aircraft and the relative terrain/obstructions. When accepting a clearance below the MEA/MIA/MVA/OROCA, pilots are responsible for their own terrain/obstruction clearance until reaching the MEA/MIA/MVA/OROCA. If pilots are unable to maintain terrain/obstruction clearance, the controller should be advised and pilots should state their intentions.
NOTE−OROCA is an off−route altitude which provides obstruction clearance with a 1,000 foot buffer in nonmountainous terrain areas and a 2,000 foot buffer in designated mountainous areas within the U.S. This altitude may not provide signal coverage from ground−based navigational aids, air traffic control radar, or communications coverage.
You can pick up your flight plan 30 minutes before the ETD and up to 2 hours after. They will come with a void time so be ready to depart at your ETD if you call before. You can file up to 24 hours in advance,
You pick up your clearance either on ground control or dedicated clearance delivery frequencies. Both ForeFlight and FltPlan.com offer GA pilots convenient access to the pre-departure clearance system that the airlines have been using for years. After enrolling in this service, and when departing from one of more than 70 approved airports in the United States, your IFR clearance will be sent via email and text message 30 minutes before departure. Flying Magazine
If you are departing from a non-towered field you can call the Clearance Delivery number at 888-766-8267 to get your clearance. They will give you a void time when you must be off or the clearance is cancelled. You can also call the number you were given by Leidos when you filed, the phone number published on the airport’s page in the Chart Supplement, the nearest RCO frequency, or ARTCC frequency.
Closing Your Flight Plan
If you are on an IFR flight plan to an open towered airport the tower will close the flight plan. If the tower is closed or you land at an non-towered airport, you can cancel with the last ARTCC frequency you were on if you can still get it on the ground, ask for a number to call before starting the approach, or call 800-WX-BRIEF.n
5−1−14. Closing VFR/DVFR Flight Plans
A pilot is responsible for ensuring that his/her VFR or DVFR flight plan is canceled. You should close your flight plan with the nearest FSS, or if one is not available, you may request any ATC facility to relay your cancellation to the FSS. Control towers do not automatically close VFR or DVFR flight plans since they do not know if a particular VFR aircraft is on a flight plan.