Here's my original sketch, which I did with a ball point pen while cooking some rice. It didn't take very long.
I then began to painstaking trace my original in Illustrator. Flaws in the original sketch soon become evident when you add solid colours .
Once I was done tracing, I refined and juggled around shapes until people looked like they were meant to. Tessa and Rachel are characters I've hardly drawn in the last five years, so I had to do a bit of revision in order to capture their attitude. Amy was easy. Apart from how she barely looks like she did then at all.
Here's the new cover next to the original. The original was never very good. I was capable of a lot better even back then, but I didn't know how to lay out an effective cover so I would often do something rather stiff-looking, just to get it done.
Despite hardly drawing anything in Illustrator since 2006, I can produce a far nicer illustration from it now than I ever could back then. It's a great program to produce a sharp-looking drawing from, but fresh hell to work in day-in, day out if you want to produce organic looking comics.