On fondant I either apply it right after I roll it out (before it crust) or sprits some water lightly on it and then apply the image.
As for buttercream, you can apply right to the buttercream, again it is easier when it has not crusted yet, but if it has crusted to much then I sprits with water.
Be careful with spritzing with water though on either medium, if you get to much water then you can get water spots on your image. If you get to much water on there just wait for to dry a little bit before placing the edible image.
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