There are two mounting holes in the back of the parts. These are used to align the parts using a piece of filament as a locater pin while the glue sets. You can download a template for the mounting holes with this link. Locate the template on your ankle, center mark the hole locations, drill a small hole in the ankle to match the filament size.
For those of you who like your droid to look just a little different than every other R2 an alternate version is available.