Okay, here's the difference between doing this professionally, and doing a "home job."
If I were doing this professionally I would never piece together drywall in this fashion.  I'd most likely have hung all of the walls as solid individual pieces.  This is going to take me a lot of time filling, taping, and sanding all those joints whereas a solid piece would take a fraction of the time... and a lot less dust in my nose.
But, I got skills...
And since I also have time, no problem.  By the time I'm done with this addition to the original closet, which, incidentally, was built like crap, Helen Keller wouldn't be able to detect a seam.
The main reason I'm posting this particular image is to show how to create really nice edges.  I prefer vinyl corner beading, but 84 Lumber only had the metal kind, and beggars can't be choosers.  This is especially true when said beggar can't afford the fuel to go into nearby Frederick, and 84 is less than 1/5th of a mile from the house.  Oh, well.
Tomorrow will be an extra special day because the power company is turning off the electricity from 9:00am-3:00pm while they replace a rotted light pole up the street.  The frigid temperatures in the house will slow down the drying time for my joint compound application, and I should have no problems reworking any potential mistakes as long as I don't die of exposure.
Before I forget, yes; that's the old door to the Guest Room.  We had to open it back up.  I'll explain later.