This doesn't have to be a true ATC (aka Artist Trading Card), but the idea is to start with a 2.5"x3.5" piece of cardstock and decorate it the way you would any ATC, then add it to your overall card.
Sound easy? Tricky?
Take a look at Jennifer's and then decide
As you can see, what you can end up with is a beautiful card!
So you can do this anyway you want, as long as you use an ATC as the general focus of your card.