I apply the mat onto the fondant after I've place it on the cake. Then I rub the fondant smoother firmly over the impression mat. I have better luck on fondant than I do on buttercream... (I use Italian or Swiss Meringue buttercreams) Hopefully others will post their tips, too.
You can use it after you have put the fondant on the cake or you can use it before. Maybe you can cover a cake pan or something and practice, see which you like better.
I prefer using these impression mats on fondant before placing on the cake as it makes a deeper impression when done this way. To place on the cake without distorting the design, I let it crust for a few minutes then use a small piece of foam (not my hands) to smooth it on the cake.
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