Cross sections, or schematics, are two-dimensional drawings that show a side view of an object sliced at a specific point. Drafting a cross-section is important for any building project. For example, if you were a designing a set for a play, you need to render a picture of how the stage will look, and then draft each furniture piece on stage with a number of views, including a cross-section, so that the carpenter knows exactly what to build.
Draw the cross-section line. It needs to be a straight line that can be viewed straight-on despite changes in shape of the surface. Or it must be traced on an object so that it goes all the way around the object and connects to the beginning, such as on a sculpture or piece of furniture. Going all the way around will help you later to depict the outside of the object and serves as guide for where it would be sliced through to show the inside in the sketch. Either way, when looking at the line straight-on, it should look perfectly straight and continuous. Label your line AB, so that A is on the left-most point and B is at the right-most point, on an individual object. This should still be the opposite extremes of the left and right. If the line vertical, A should be at the top.
Draw a frame for the cross-section. The frame should have three sides: left, right, and bottom. The bottom line represents the same line from Step 1 as overall width. The lines on the sides are there to help you with height. Your frame needs to be in the same scale as the drawing you are working from. If you're working off of an actual object, or an imagined object that you're designing, you need choose a scale that will give the best representation of the object possible. Set designers work with a scale of 1 foot equaling 1 inch in the drawing.
Use the frame like a graph to draw points along the cross-section line. You may want to start by drawing equidistant points on your line from Step 1. At each point, determine the height of that point and plot it in on your cross-section frame by measuring the distance of the line on the cross-section line so that it's the same as on the base line, and then measure vertically on the frame to the correct height. If there is a significant increase in height, or change in contour, draw in more points along this part so that you end up with the most accurate schematic view possible.
Connect the points from left (A) to right (B). If you're drawing a schematic of an object, such as a vase, think of working clockwise instead. This way you can get the outlines of all crevices so that the onlooker can see the outline of the entire shape, not just the top and sides.