Re: How to plane an old door? I'm a rookie...
You can use a router and a straight edge to take the 3/16" off, do it at about 1/16" at a time. You could use a plane, mark a straight line and plane to it. Or you can use a belt sander, mark a line and sand to it, this will take the most time. If it's a solid door you can use a straight edge and circular saw. A steel stud ( available at most home centers works well as a straight edge. If you use a circular saw get a good 60 or 80 tooth blade and mask the cut line to reduce tear-out.
Be sure you live your life, because you are a long time dead.-Scottish Proverb