Good point. I have replaced a household full of doors. Start with the knob location and trim top and bottom as required. Also, do not assume the opening is square. Use the removed door as a template if possible. Fillers can be installed as previously described. I fabricated fillers to the required thickness, cut to length, and glued in place. Use clamps with a scrap across both sides of the door to distribute clamp pressure. Let the glue set overnight and trim as required. You've only got a single door but it gets pretty easy after a couple. Good luck.