I have found that the impression mats really are not very effective on soft buttercream.
If this is a test cake, give it a try. Chill the cake to firm up the icing, and quickly try the impressions.
Usually, the mat tends to stick to the frosting. You can make a patterned impression with a ruler, making sure it is clean and dry between imprints.
Maybe someone else has a better suggestion.
I have used this impression on some buttercream cakes w/ good results. I do wait until the bc has crusted. Then I take the diamond impression and sift some powdered sugar very lightly onto it...so it is less likely to stick. Then I go around my cake. I don't press firmly at all...just enough to leave a slight impression. After about 2 impressions I sprinkly some more powdered sugar on it. I have never had a problem w/ it sticking doing it this way. My only issue is getting the lines to match up at the end.
I posted a question about this not too long ago. I was told that the impression mats do not work well on a non crusting buttercream icing. Another tip that was given was to wait until the icing has crusted and use a little cornstarch before applying the mat.
After messing up my cake(3 times!)...I have not yet tried the cornstarch! 
traci,
Thank you for your input . I figured out that if I pop the cake into the freezer for about 10 minutes and let the icing firm up, using the impression mat is no biggie and actually works great dusted w/ a tiny amount of powdered sugar. You may want to try this method. It worked wonders for me AND kept me from ripping my hair out 
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