remove the hinge covers from the door pillar, look inside the hinge appature with the door open and there are locking bolts (not always fitted) loosen or remove then close the door not fully, lift vertically and pull forward when the balls release out of the sockets on the car, much easier with the front door open , 2 people make the job easy one on the back seat and 1 outside lifting, depending on rust you may have to work the door open and closed to work the balls out the socket but be sure to only move the door an inch or to in and out as there is a stop on the hinges that limits its travel when fully open, if this starts to engage with the inner skin of the c post the door will not come off