First glue the joker's hair onto his head and do the same with Batman's Helmet. Make sure they are both exactly centered. You should only need a drop of gorilla glue to accomplish this.
After the glue has dried you have the option of gluing the body together. I decided not to so i could wear them as just heads or the or as the whole figure. I found that even without the glue the minifigures stay together well.
Next you take a drill and with the smallest drillbit you have drill a hole directly in the center of the head. I used a drill press and highly recommend you use one too. If you do not own one then have someone else hold the minifigure as you drill
After the hole is drilled cut your eyepin to the size desired.
Put a drop of gorilla glue into the hole you drilled and stick the eyepin in. Hold it in place until the gorilla glue dries.
Now attach the earring hook.
Repeat the process with the other minifigure