The way I go about making barn doors is relatively simple (or else I couldn't do it!).
I measure the opening that I want to cover, and decide if I'm gonna use one or two doors. I usually use big swinging doors, but we've not a few sliding as well. The door construction is essentially the same, but the sliding doors will be a few inches wide for a given opening because you probably want an overlap on the side.
Example: The opening for the barn is 10' wide x 10' tall. For swinging doors, I would make a set of doors frames 4'-10" wide x 9'-10" tall (you will have a space in the middle, but you can cover it with an overlap of metal).
Anyway, decide how big you want the door. I usually use 2 x 4s for the frame (although for a really big sliding door, we have used 2 x 6s. Then I cut the outside frame members with 45 degree cuts on each end (basically, make a mitered frame). Lay them all down on a flat surface and put a screw through the ends. Take a few minutes, measure the diagonals, and square the frame. Then I cut a board to bisect the big rectangle, and make to small ones (put it equal distance from each inside end). Toe-screw it to the outside frame.
After all that, I cut diagonal braces to run from what will be the inside bottom and inside top corners of the door to the middle of the middle (that really doesn't seem to make sense, does it). Anyway, you will probably have to cut and recut and test fit some times.
After all that, you should have a frame laying down that looks like a digital "8" or possibly upper case "B" with half an "x". (Maybe I should just make a drawing and post it). After checking for square again, I put metal mending plates on each junction of wood. I use the kind that have a bunch of holes and just put 1" drywall screws through the, at least 3 per board. This will be the inside of the door.
After that, you have built a braced frame. You can now stand it up in the opening. I usually use cut off chunks of wood and brace it up how I want it and then attach the hinges. If it is a hanging door, you will need to attach the roller hangers to the top of the door (usually have to drill through the top member) and then slide it in the track.
Finally, after all that, I hang the metal on the door. On sets of swinging doors, I start in the middle and make sure that the ribs of the metal covering from each door will overlap in the middle, and cut the rest of the metal to fit.
Wow - that looks really convoluted. It's really not that difficult. If I get a chance, I'll post a sketch of what I'm talking about it should be a lot clearer.
Good luck. Take care.
