Control templates are a complex concept in themselves and so editing them can be a bit mind-bending for the first time. If you’ve not encountered the idea of templates then perhaps the simplest thing to do is to wander into Blend and make a new project.

and then drop a simple control onto the artboard. A Checkbox is a prime example and a Button would be another. I’ll go with Checkbox.
and then wander into its control template – i.e. the visuals that represent the control’s look and feel. The ways in which you can edit the template are numerous but I think they all involve the same 2 menus which I’ve almost managed to show on this screenshot below;

Read more: Mike Taulty's Blog Part 1, Part 2