This is totally up to you as I see many different eyes presented every year. Some people enjoy the candy eyeballs but I like sticking with ketchup or mustard. Mustard will show up much better but ketchup can look nice too. I dip a toothpick in the sauce and then stab it through the hotdog, just a little bit to create the eyes. The sauce covers the holes and the eyes stay in place because there's not too much in one spot.