An image showing a method for attaching Harley's Heads is below. Also, an image to suggest at least one way of coloring the characters is included as well. In the comic books currently, both characters have white bleached skin (as opposed to the makeup The Joker used in the film). Also, Harley's hair was originally blonde, but depending on what depiction you are looking at these days, she wears it dyed a variety of colors/styles.
I've also included a sequence from Batman: Legends of the Dark Knight (no. 142) in which the Joker decides to take a bath in Ra's al Ghul's Lazarus pit, which is where I got the idea for the doll's underwear. The only thing I changed was that they are sad faces in the comic book. I didn't realize that when I drew mine. May have to go back and fix it.
The Joker was created by Bob Kane & Bill Finger and is the property of Warner Brothers / DC Comics. Harley Quinn was created by Paul Dini & Bruce Timm and is the property of DC Comics. These dolls are intended for your personal amusement and should not be sold for profit or broadly distributed.