Chalk lines are the fastest way to mark straight guidelines on flat surfaces. They are used extensively in the building and construction industry to speedily and accurately 'draw' straight lines between two points... Secure the string to one side or corner of the area you want leveled. Attach the line level to the middle of the string using the small hooks on the line level. Pull the string tight while sliding the level up the string within eyeshot. Pull tightly enough to avoid any sag in the string.