I tend to take Halloween very seriously.
In here you'll find how to make the costume and do the makeup. I encourage you all to take the time and read through the entire Instructable before beginning, just so you know what it is you're getting into. There is sewing involved, and I'm going to go ahead and write this as if you've all done some sewing before but PLEASE if you haven't done any sewing or aren't sure of something, ask! I'll do my best to help you out. In total, I spent about 11 hours on the costume, but I did have some issues that I'll explain later, so if you're familiar with a sewing machine and how to put a pattern together it shouldn't take you nearly as long.
The costume bits start on the next step, and the makeup starts on step 8.
Last, but certainly not least, if you've found this Instructable useful in your Halloween pursuits, or just liked it at all really, please take a moment and vote for it in the Halloween Easy Costumes Contest! I would appreciate it very much!
Now, on to making yourself the Joker's Hench-Wench, er, girlfriend.