Ideally the flight plans for the INS system are created using FSNavigator. It can be done with the regular flight planner as well but it is a bit more tricky.
Be sure to read the INS documentation for that part. Or just have a blast with the old gal spraying the heck out of some FS forest.
Not one to leave things incomplete Volker is already strapping spray bars to the bigger sister...she will be a hoot as well.
Cheers
Stefan