Sorry for the late response! My method for this one was fairly simple:
1. Pencil on paper, then inked with a Faber-Castell Pitt Art Brush Pen.
2. Scanned it, made an "ink" layer with all the line art and a "color" layer below in Photoshop.
3. Dropped in a simple background with my Cintiq, then added the bubble effects.
If there's anything more specific you'd like to know about it, just drop me a line.