I sliced off the rounded top of each cake. Turning them upside down would not have worked; I wanted both top and bottom to be flat.
I cut the top of four cupcakes off. Leaving them in the wrapper made a perfect guide line for me.
I laid the cupcakes, upside down on the cake to test out where they would go and make sure they fit.
Then I iced the base....
...and each cupcake. I tried holding the cupcakes from the bottom and icing them, but I could not hold them steady enough and I got more icing on my fingers than on the cupcakes. So, I cleaned the bottom of an icing can, put a dollop of icing on it to use as "glue" to hold the cupcake in place and iced them that way. So much better!
When i had completely iced a cupcake, I used a thin, metal spatula to slide the cupcake off the bottom of the icing can and onto the cake.
I could have left the cakes like this, but that' not me. Every nodule (they are actually called "pips", according to LEGO) on every LEGO brick has the word "LEGO" in it. So I used a toothpick while the icing was still soft and wrote LEGO on top of all 8 pips.
There you go!