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We reused old floorboards which had been taken up to replace. You don't need to router when working on the face of the door. You can just use a sharp Stanley knife to cut out the edges. If you can I'd try cutting your edges so that they have a draft angle, it makes it easy to jam the pieces in. Its not a difficult job to learn as you go.
Just adding, you might find it easier to remove the mortice lock and fill with wood and use the type of lock mounted on the door.
I did wonder about that kind of thing but it might be a bit beyond my skills. Did you route out the old? Is the new just glued in? Any tips for matching the wood? Not sure my parents Victorian doors are the best thing to learn on though.