To understand how to draw a cross section line, first you have to be assure about the line to be straight. This is necessary from analysis point of view of structure, regardless of changes made to the shape of surface. Other way to draw it is by letting it go throughout the object and again joining the start point. This will help you tracing the situations where you need to describe boundaries of object and also to know from where object should be cut or carved. Straight line you drew must be flawlessly straight and continuous in nature. Label this line as PQ, so that point 'P' is on left - side of point and 'B' on right - side for individual object. 'P' should be on top when line is vertical and similarly point 'Q' should be marked opposite to 'P'.
Next we draw a face for cross-section. This face should contain three sides: left, right, and bottom. Bottom line is the one you drew earlier and name it as PQ. It is called as width of cross – section. Other two lines represent height and length of cross section respectively.
This face of cross – section seems to be like a graph which can be used to draw points along cross-section line. Draw equidistant points and determine the height of that point which has to be plotted on your cross-section frame. Connect these points from left to right to get your desired cross section.