I think you might be the one that Bear Foot and I have been having a conversation about how to do this. There are so many different ways you could build it that I can’t come up with a list. I don’t know what tools you have or what materials or techniques you are planning on using.
One thing I suggest is to draw it out, to scale if possible. First an overall look at it, maybe from the side and the end view, but also some close-ups of your connections. Your connections are probably what are going to cause a few extra trips back to the hardware store. That happens to all of us.
A lot of lumber comes in 8’ lengths. If you carefully lay it out, you might find you do a lot less cutting and have less waste and quite likely no additional expense if you build it 8’ x 16’ instead of 15x6. You have to pay attention to out-to-out dimensions versus centerline dimensions though. That’s where sketching up your corners and how they go together can be really important.
I don’t know if you are planning on building your own door or getting an outside door, maybe off Craigslist to save money. I don’t find doors that hard to build but they can be intimidating if you haven’t done it before. One trick is that they generally need diagonal bracing to keep from sagging or warping.
I’d suggest you use screws instead of nails. They are easier to take out if you need to correct something, I think they hold better than nails, and they can be easier and safer to install. I use a drill to drill a small pilot hole when I use screws. That makes them a lot easier to install and it helps keep the wood from splitting. If you are using treated wood, get screws made for treated wood. They’ll last a lot longer.
Since you say you are not carpenters, you might want to go to Home Depot or Lowe’s and look in their catalogue section. They might have a book there that shows you how to do a lot of this stuff. Or even better and less expensive than buying a book, go chat with your librarian. Most public libraries have books that show you how to build doors or build the other stuff or they can get the books for you to check out. Not all librarians are equally good but their job is to help you find that book.
I know this sounds intimidating and yeah, you will make mistakes, but it is something you can do. Good luck with it.