Do you use a toothpick, wire, what? Thanks in advance for your help.
are these gumpaste or real?
Real callas are toxic and should not be placed on a cake at all.
If you are talking gumpaste or fondant flowers, you can use either or none, depending on what you are doing with them. You can make the center part and insert a wire or toothpick. You also can easily make them with no wire or stick and just arrange them flat on the cake.