There are 3 doors in the building, 2 garage doors and one regular door.  The regular door is now dedicated to the storage room, so I have no way to get into the main shop area without rolling up the garage door, so I decided to install a dedicated, locking entry door.  My neighbour the general contractor measured and ordered a door for me from Atascadero Door.  After I picked it up, I pulled off the drywall and determined which studs to cut and where. Then I framed in the new opening.  Since it's a load bearing wall I did a double-thick 2x4 header.

Then I cut the opening in the exterior wall, dry-fit the door and determined how far to inset it, where to set the tread and how off level the side of the building is (a lot!) I then installed the door with shims, and sprayed expanding foam in the gaps.  Yesterday I went to the Templeton Habitat ReStore to get my volunteer orientation and while I was there I spotted a still-in-the-package Schlage exterior lock set, and grabbed it for $3. It went in perfectly.

And now the door and trim are fully installed and painted.