This took me WAY longer than an hour and a half, but I'm happy with the outcome. I tried to follow Karla's process pretty closely.

Floral element: Sea plants
Location: Under the sea
Time of day: Night 

Composition sketch. I wanted two focal points — the mermaid and the octopus — which I tried to spotlight by using the rule of thirds and by having everything sort of lean into toward them. For the mermaid, I tried to mimic the character pose from Karla's drawing:

Refined sketch:

Color sketch, trying to follow Karla's advice to set the focal points apart with that red color I'm not using elsewhere:

Color block, but the further along I got, the more I realized I didn't love the gray, or the purple tail, or the yellow plants, which I thought drew too much attention, or the night sky being so pale, so this changes in the next one:

Color sketch 2, I was happier with the yellow mermaid tail sort of matching the yellow details on the octopus. I changed some of the plant coloring, too, and roughly sketched in some details on the mermaid, the sky, etc.: 

Shading, detail on some plants and the octopus, and I wasn't happy with the mermaid's tail, so I took another pass:

Final version, with some additional plants that I think helped balance out the colors, different eyes on the octopus to hopefully look more friendly and better match the mermaid, more shading, more texture, some color adjustments, and sky details:

 

Thanks for a fun class!