Start with a line, then decide what operation you want to do to the line. This will probably result in a figure with more lines. Repeat the same operation on each of the lines of the new figure. Now, lets use the following set of figures to describe an actual process. We start with a line.
Lets add a square bump in the middle of the line.
To make it keep the same overall size, the length of the lines used to form the bump are one third the length of the original one. By looking at this second iteration it might be pretty hard to figure out what the axiom and production rules are. You may want to try to draw the fourth iteration using ruled paper. It is pretty hard to keep track of where you are. Now imagine trying to do a few more iterations.