Click the "Start" button to run it. Click "Reset" to clear the screen and "Randomize" to fill it randomly with different types of cells. Click "Draw Circle" to draw a circle somewhere on the map (usually circles are a good starting point for more complicated structures). Click "Reset Conductors" to reset all head and tail cells to conductors. Each cell can be modified by clicking it.
This is a self-contained HTML page, so it can be downloaded and modified (in particular, the parameters in the script can be changed). I also wrote a command-line version in Python which can be found here. Both were written after midnight, so take the code like you take your password hashes, with a grain of salt.