To paint, click the Canvas with different dyes. Right-Click and drag to draw lines and curves.
You can also double click with a Bucket and a dye in your offhand to fill in an area.
To darken pixels a right-click the pixel with coal, to lighten pixels right-click with a feather.
To flip the painting horizontally left-click it with a compass, to flip the painting vertically right-click it with a compass.
To grab a color left-click the pixel with a sponge, right-click with the sponge to paint with the color.