They started by painting from dinosaur maquettes to get used to visualizing a creature in a setting with believable light.
Then they moved on to mermaids. The students looked at how mermaids have been portrayed in art history, and then did lots of sketches from their imagination.
They brought in a female model to hold a mermaid-like pose (for the upper half at least).
Patrick O'Brien bought some fish at the market so the students could do empirical research on the mermaid's lower half.
They put it all together, with lots of drawings and studies to fit the mermaid into the environment they imagined.
They did their final paintings in oil. Not many students get a such a rare chance to paint fantasy creatures based on real-life inspiration.
Imaginative Realism on Amazon
Previously: Satyr Sketching in China
Related story on NPR: Should we create human-animal genetic hybrids?