Andy's dads way is the traditional way of planing the end of a door with a hand plane, however I would not recommend doing it that way with an electric plane. As you have to stand astride the door holding it in place with your knees, one slip and you could find yourself planing your knee caps. The only safe power tool method in my opinion is to use a router. I have a Tee-square like guide which I clamp to the door with the Tee part held against the long edge at the end of the cut with a sacrificial piece of timber held in between. This produces good edges without splinters, the only problem I have encountered with this method is the tendency on some cheap panel doors to use staples to hold the framing timbers together during manufacture, but these are a problem whatever method is used.

Richard