As a first time installer you might find yourself questioning the integrity and or quality of manufacture as you reach ¾ height on the walls !
Depending on the design, you are likely to have the side walls adjacent to the front wall (with the door in) flaring outwards. Never fear, this is a temporary trait until the walls reach full height and it all becomes tied in.
Below shows how this manifests itself. The correct line of the wall is indicated by the red line. You can see the wall beyond the door opening is currently flaring out of alignment shown by the curved blue line.
As you reach full height, a full width log will span across the side walls and over the top of the door opening. When settled into its correct position, it will ensure the side walls are aligned to their correct width and the front wall aligned vertically.
If you want to get the walls perfectly vertical, use a spirit level, best to be at least 4ft (1.2m) long and place it up against the logs. By using a soft faced mallet, you can tap each log to achieve the desired affect.