So, you're about to hang a picture, or some shelves, or in this case, a wallpaper mural. First step is laying out the measuring and that requires a measuring tape, a pencil, and something to keep the pencil sharp, except not today, because today, we're using a self-marking tape measure with a built-in marking device, because it's so much easier.
The first step requires measuring the wall space which is 14' wide by 8.8' high vs the size of the mural, which in this case 12' wide by 8" high. Plus, to make it tricky, the wall is in an old building and the ceiling is 1.5" out of level from the left to the right side. The solution is to burn 1.5" at the starting point on the left side, so we can keep the panels plum (meaning vertical, and not crooked). So we mark the bottom location of the first panel so that we can trim 1.5" off the top after the panel in up, to match the ceiling moulding line, and we're centering the mural to the wall so there'll be approximately 1 foot of painted wall showing on each side.