If you have a third hinge (typically used for heavy doors), it will be placed directly in the middle of the other two hinges (this will put it slightly off-center).
Using a dull chisel will make it more difficult to cut the mortise, and cause you to use more pressure with the hammer (which could cause a slip). If you cut the mortise too deep, you can use wood filler or place a thin piece of wood in it before installing the hinge.
If the replacement hinges are sized differently from the old ones, fill in the mortise holes in both the door and the door frame with wood putty. Use a metal scraper to fill in the screw holes or plug them with a small wood wedge. Allow the putty time to dry, and sand it smooth and flush with the surface. Paint or stain the repair to match the rest of the door and trim.
If you have a third center hinge, you should replace it at this point as well.
