How do I file a Defense VFR (DVFR) flight plan?

To File a DVFR flight plan, you will need to use the ICAO flight plan form and enter your ADIZ penetration time and location in the REMARKS section on the ICAO form (Equivalent to line 18 - Other Information on the printed ICAO form).  

Here is a more descriptive explanation of how to do this:

All aircraft entering U.S. domestic airspace from points outside must provide for identification prior to entry or exit in out out of an Air Defense Identification Zone (ADIZ). ADIZs have been established to assist in early identification of aircraft in the vicinity of international U.S. airspace boundaries (AIM Section 6, 5-6-1). 

Many aircraft inbound to the U.S. will cross an ADIZ. There is no ADIZ between the U.S. and Canada. According to FAR Part 99, if penetrating an ADIZ, all aircraft of U.S. or foreign registry must file, activate, and close a flight plan with the appropriate aeronautical facility. In addition to normal ADIZ position reports, and any other reports Air Traffic Control may require, a foreign civil aircraft must give a position report at least one hour before ADIZ penetration, if it is not more than two hours average cruising speed from the U.S.
For Defense VFR (DVFR) flights, the estimated time of ADIZ penetration must be filed with the appropriate aeronautical facility at least 15 minutes before penetration, except for flights in the Alaskan ADIZ, in which case, report prior to penetration. Additionally, VFR pilots must receive and transmit a discrete transponder code.
Be sure to activate your flight plan before crossing the ADIZ.
ICAO VFR flight plans must include in the Remarks section of the flightplan: DVFR estimated United States ADIZ penetration at time (UTC) and estimated point of penetration (latitude and longitude or fix-radial-distance).
DO NOT include the "/" or "-" or "(" or ")" characters as this will result in a rejection of the flight plan if used in the remarks field of an ICAO flight plan.
Here is an example of how a border crossing DVFR flight plan remark using latitude and longitude would look:
     DVFR 1400UTC 2728N 9929W  
or using a VOR:
     DVFR 1400UTC LRD242004
See also regulation text eCFR 99.3 and eCFR 99.11