First, load your image of the Arecibo grid to use as a background template:
You may also need to manually set your Grid Dimensions and press the "Set Grid Dimensions" button if it doesn't align with the default values.
Then, click over the top of the image to generate the binary string, by painting in the cells with mouse left-click/drag. The default value is 23x74 because initially this was used to create the binary string for the 2001 Arecibo Answer, which was a 23x74 grid.
You can also set your own custom binary sequence to generate the grid from, by pasting it into the "Binary String" textarea and pressing the "Manually Refresh Grid" button.