(We hope to make things more intuitive as we revise this. Meanwhile, please read before proceeding.)
Think of the area on the left as a stage, and you are offstage giving directions. You call for a new "actor", such as a line or a circle, to appear on stage, and then you give that actor stage directions to position it the way you would like. But you do not physically enter the stage. On the left side of each actor is a blank space to enter a name for that actor. Names are optional, except for labels.
Here is an example for creating a Venn Diagram. Following these instructions will help you learn how to use the diagram editor.
On stage, we will have a rectangle to symbolize the universal set, one circle to symbolize set A, another circle to symbolize set B, and two labels, A and B, to label the circles.
Our first actor will be a rectangle. You have the option of naming this actor by typing in, say "universe" in the blank. But we could just leave the actor's name blank. Then pick "box."
Next, go to the right of where we picked "box" and click "stage directions." This will cause a set of checkboxes and buttons to appear. Let us move our box closer to the center of the diagram by pushing the "big shift up" button a few times. We might also make our box wider and taller, by pressing the appropriate buttons.
Underneath the stage directions, we have a new row of choices for our next character. To get a circle, we click on "ellipse/circle." Then, we push its "stage directions" button. We check the box to make it a circle. Then, we use the button controls to position it inside the box, toward the left side.
We now have a new row of choices for our next character, which will be another circle. We pick "ellipse/circle" and then push its "stage directions" button. Again, we check the box to make it a circle, and then we use the control buttons to position our second circle inside the box, toward the right side, with some overlap with our first circle.
Now, we add our labels. Now we must type something into the blank on the left of our new row. Type "A" there. Then click "label." Then click its "stage directions" button. Now, click on the larger font size button a couple times to make the text larger. Then use the control buttons to move the label into the middle of the left circle.
Now, in the new row of actors, type in "B", select label, and give it appropriate stage directions.
We are finished with the Venn diagram, but try playing with the other characters. Try adding a line. You can rotate it by changing its slope. Since a slope of infinity would be problematic here, you get a vertical line by checking "vertical line." To get a horizontal line, you can just keep reducing the slope until it gets to zero (you must not have "vertical line" checked).
The other character that you can practice using is a point. A point is pretty simple, although note that you have the option of making it big or not big.