Choose a second colour, different from that of your colourful background.
Using the dropper from the bottle of acrylic ink, apply a single drop of ink on your paper, at the place you wish.
Then use a paintbrush to spread the drop of ink to create a long brushstroke on the colourful background you made.
*Hot tip: for the ink to make longer brushstrokes, use a slightly wet paintbrush.
You can also use a paintbrush to spread water on part of your paper and apply a drop of ink to the wet area.
Then, watch the ink slowly spread as if by magic!