How a deck will come together is determined even before the first hole is dug.
When you start to measure out your deck, you need to ensure that all your measurements are taken level. I know this may seem obvious, but doing this on a steep site like the one in the photo is not always easy.
Looking at the photo, you may have noticed that the profiles (the temporary timber frames that the string lines are tied to) have different level steps in them. When the profiles have steps in them you are able to transfer measurements between the different steps, while at the same time keeping all the measurements level. Also the stepping process keeps all the profiles at a comfortable working height and makes it much easier to install your footings later on.