Start by cutting out a bunch of modeling chocolate pieces using the same cutter as the one that you'll use for the cookies.
You can put these aside to wait for the cookies to bake.
When the cookies come out of the oven, take the modeling chocolate pieces and put them directly onto the hot cookies. Don't wait until they cool off, you want the chocolate to melt onto the cookie so that it sticks when it cools off.
Now for the important part...DON'T TOUCH THEM WHILE THEY COOL OFF!!! You'll be tempted to test them, but don't. The chocolate will soften up and get oily-looking and melty, but it will solidify again when the cookies are totally cool. If you touch them while they're still soft you'll leave fingerprints and oily marks and you don't want that.
When the cookies and the chocolate cool off completely the chocolate will be a solid surface ready to decorate. You can pipe on icing or use wafer paper as shown in the following video:
And if you've never made modeling chocolate before, it's easy. I have a video for that, too :)
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com