First git a plane, any plane. then fly near the center of the map and go straight up(boost helps). slow down when ur realy high(not to high or ull die)watch ur hight meter or go on the fly-by view. when u stop just before u fall back hit the eject key(default 'E') and u will fall, use the mouse to steer urself around in the air, look around for ur plane and fall near it. dont run into it or ull die, when ur dun flying around fly over close to ur plane and press 'E' or whatever to get in and fly off. dont get carried away tho cus the ground cums up fast and ur engines need time to start up(they turn off when u hop out). also when u get low enuf then u start falling faster than ur plane.
PS. if u had a friend who cud do this as well u cud try switching planes in the air. :)