You look at what you already have and find a way to re-use it. I had a big pile of 100 year old lathe that I had saved from a bathroom remodel and a crazy desire to cover that 1970's disaster.
First, I put on my headphones and spent some time pulling all of the nails out of this beautiful stuff. This takes a while so check that you are fully charged before starting this step. Once the nails were out I assembled the table saw and nail gun and got to work. I guess you could measure, but I really found no need. I started with a piece, gave it two fresh cuts on either end and nailed it up, starting in the bottom left corner of the wall. For the next piece, I gave it a fresh cut, placed it next to the one I just put on the wall and marked where to cut it. After cutting it I just nailed it up. I was careful to stagger the joints so they do not line up. Really, this is not rocket science and is pretty much foolproof.
While not difficult, this process is time consuming. I did this portion and the side to the left one day, and the section over the closet doors the next Saturday. It doubles as a gym workout in my book as I went up the ladder like 100 times.
When all is said and done this is an easy project with a lot of impact. I used lathe, but wood from old pallets, or fence or anything could be used. If you don't have a nail gun,
put it on your Christmas list, you can rent one from your home center along with an air compressor.