Click a color button, paint away. Make sure to change the grid size and unit size before painting, changing these values mid-painting will re-create the artboard and your progress will be lost. If you want to use an image as a template, drag and drop the image onto your artboard, you should then be able to toggle it on and off. When you click 'generate div', a single div is created with a css block applied to it to render your artwork. It's also copied to your clipboard just in case you want to use it elsewhere.