For each lens you want to add, first click the "Add Lens" button and then click on the canvas to add the lens. The default is a positive (converging) lens. To add a negative (diverging) lens, first click on "Lens Type" to toggle between "Positive" and "Negative".

To add an object, click on "Add Object" and then click on the canvas for where you want to place it.

Each lens will come with a default focal length, denoted by the lens number. To change the focal length, just click down on the number and drag it around.

You can drag the lenses around by clicking and dragging inside the lens, and you can change the vertical position of the optical axis by clicking and draging the axis.