I started with a loose pencil sketch (very, very loose!) and jumped straight into working with ink brush pen and traditional ink and paintbrush to create the fluid looking linework. I then scanned the artwork at a high resolution and worked up the colouring on Photoshop with custom brushes and watercolour textures. It's one of my favourite ways to work quickly as there's so much potential to switch colours around and make easy adjustments.
I'm considering making a small print run of these available (and potentially have the original artwork up for grabs in my shop), so give me a shout if you'd be interested!