I started by attaching everything with masking tape, rolled on itself so it was sticky on both sides. You shouldn't use the masking tape for anything other than the flat pieces, but I didn't know that yet.
As soon as I got up to 20MPH on the street, the ears (which could not have been better shaped to catch the air) went, flopped and flatted. Fortunately, I had brought double-sided foam tape, so I used it instead and they kept it well for the rest of the day.
The flat pieces (eyes and teeth) were good all the time with the masking tape. If you want to drive on the highway with ears or something similar, you may want to find a safer way to attach them, or somehow make them more aerodynamic. For 25-35 MPH, however, double-sided foam tape worked well for me.
In any case, use pieces of double-sided foam tape on the tabs you design in your templates to attach them to the car.
Unfortunately, I did not intend to get a close-up view of how the tabs are attached to the car. Duh! However, to get an idea of what I'm talking about when I say "tabs," search online images for "cardboard attachments."