The fact of the matter is that when people sit down, they can become more difficult to draw for some people. When new variables are introduced to the drawing, like a chair and foreshortening, drawing a seated figure can be intimidating. However, if you remember a few simple steps to drawing a human figure, the difficulty can be drastically reduced.
Keep in mind that are no formulas for drawing a person. There is, however an order that you can construct your drawing in that will result in better results.
First, draw a line from the head to the feet of the person that you are drawing. Since our person is sitting down, this line will probably bend at some point. This is fine. The purpose of drawing the line from the head to the feet is to be sure that we get the entire figure on the paper and to begin to think about our person's proportions.
Next, you will need to determine where the shoulders and the waist of the person will be and draw a line to indicate each. These lines may be diagonal, which is okay.
Now, you can draw a simple stick figure. From here, we'll finish the drawing.
The featured video demonstrates the whole process of drawing a person sitting down.