Correct way to plane a door ?

Mark cut on both sides acurately then cut with circular saw using a guide clamped to the door, hand saw is good if you are, but a jigsaw is crap for the job.

If you must use a power planer and aren't expert set it at its minimum cut and take lots of cuts across the whole surface, go both left to right and right to left but stop before you go off the ends to prevent ripping out the end grain. You can clamp or screw a couple of blocks to the door edges to extend the area the plane will be cutting so the end grain is held in place on the coners as you plane, that way you can cut from end to end. Every couple of passes look to see what bias you are putting on the cuts by checking with a set square all across the base. Once you know where you are cutting too much or too little try and compensate. You can also use a couple of straight battens clamped at the finished level and plane down till you nearly touch them.

In answer to your question start at corners and work in unless you can protect the end grain.

But my advice is don't use an electric planer - others would say different.

btw what sort of door?

Just been outside working - and thinking - and it occured to me that possibly the best reason for using a saw and not a plane of any type, is the angle you are working at.

With the door flat on a couple of trestles or a workmate you are at the correct angle to use a power saw or hand saw, you are 90degrees away from the best angle to work with a power plane or jack plane.

There is also a tool you can hire (sorry UK site) for doing the job WITHOUT removing the door from the frame . Probably only worth hiring if you had several doors to do; essentially its a circular saw that is designed so it can cut very close to the floor. Bit messy to use as you have to clean up inside but very quick way to do the job.

btw I've used quite a few power planes and the really powerful ones (even some of the ones from Argos) will remove timber at incredible rate, so if you are pushing slightly harder to the right or left when you use it after a couple of passes you'll be way off having a straight edge to the bottom of your door.

For best results I'd put the door on a couple of trestles mark the cut then set up a batton and clamp or screw it in place for a power saw to run against as a guide for the cut.

I have it in the back of my head somewhere an old rule along the lines that if you are removing more than twice the thickness of a saw blade (over say 4-5mm) then its a saw you should be using.