Okay, you guys know how I'm not a cake decorator, right? Like, you've been here through all of my rants about how the Cake Boss and similar shows have made people think baker = decorator when most decorators aren't actually great bakers. (I know a lot of cake decorators who use box mix because it's easier, quicker, and they can spend more time decorating. I'm not hating, I'm just saying that you don't have to be both) I know with me, I really don't have the patience for most cake decorating. Occasionally, I'll go a bit manic and decide that I can make royal icing transfers or dalek cupcakes, but even then, I'm aware of my limitations and try to keep things simple.
So when I tell you that that cake up there was super easy to decorate, you'd best believe me.
Also unlike a RI transfer, if a frozen buttercream transfer cracks or breaks while moving it, it's no big deal. Let the buttercream soften for a minute and then press the cracked pieces back together, smoothing it out gently with your finger or a piece of waxed paper. Mine broke into three pieces when I moved it to the cake. Can you tell where it happened? Cause I sure can't.
If you're using a physical coloring book, you can scan the image in and flip it, or you can trace the image on tracing paper with a sharpie, then just flip the tracing paper over.
Once your outline is finished, stick it in the freezer for about 15 minutes to set up. This is not strictly necessary, but it will help prevent bleeding and makes it easier when you fill in the rest of your colors.
While your outline is chilling, you can go ahead and mix up your buttercream. You don't want to use store bought frosting as you need the butter for it to freeze firm, so you'll want to use a recipe that's at least half butter (half shortening). If shortening skeeves you, I see no reason you can't use all butter, it will just need more time to thaw, and it won't take color as well.
I used 1 cup unsalted butter, 1 cup solid white vegetable shortening, 1 tsp. vanilla bean paste, a pinch of salt, and I didn't actually measure the sugar, but it was probably about 4 cups.
Now, I specifically chose this design to cover the whole top of the cake, which is why I needed fill in all that white in the last picture. You don't have to do that. It depends on your design and your preference. I just like to do it this way. If your transfer is not going to take up the whole cake like mine, you'll just need to use your background color (in my case, white) to fill in any gaps (like the holes in the "hoot hoot"). Stick the transfer back in the freezer for another 15 minutes.
The last step, which I forgot to take pictures of, is a layer of more white (or whatever your background color is) over the entire transfer, making sure it's smooth and even. This makes sure you have an even base for the transfer. If you skip it, the frosting might sag once it thaws, and that doesn't look good.