I have a pass through where I want to install a door. The existing space for a new door is roughly 90'x30, but it wasn't built well, so the width varies top to bottom from 29-30". I'm thinking of installing a custom made 27-28" wide door and building the frame around it. How do I install the frame to the existing space and account for the 1" gap between the top to the bottom? There are metal studs surrounding the opening.
Personally, if it were me, I would buy a prehung 28" door and install it there. Less expensive and much faster than a custom door job.
To determine which swing to buy (left or right), back up to the hinge side that you choose and whichever hand is on the side of the way that you want to door to swing, get that.
You can shim out the framing with 1X4, as necessary.
To install it, you will need a level, a carpenter's square, hammer and/or screwdriver, and beveled shims (smooth cedar roofing shingles work great).
To make sure that the door swings clear of the floor, raise the prehung frame up on blocks (1X or 2X, depending on how high it needs to be raised in the frame to the header.)
Remove the door from the frame, and hang the hinge side first, making certain that it is plumb and square before nailing it. Push two beveled shims in from opposite sides behind the hinges to plumb it. Then do the top, making sure that it is level and square, and then do the latch side plumb and square.
The bottom of the door should swing 1/2"-5/8" above the finished floor (carpet, tile, wood, etc.).