I'll give a brief description of what I did, but the best explanation I found for how to do a frozen buttercream transfer is on the Wilton website.
Frozen Buttercream Transfer
Buttercream Icing (recipe here)
Desired design outlined on paper
Cookie sheet or flat board
Decorating bags and tips
To start, you'll need your design outlined on a piece of paper. For this cake, I measured the width and length of the picture frame I wanted. If you're doing a character for your cake, you'll want the reverse image (just flip over what you've drawn and trace it).
Tape the waxed paper down over your drawing. I taped both the drawing and the waxed paper to a cookie sheet so that I could easily move it all to the freezer later.
Take a decorating bag filled with your outline color and trace your design. Since I simply wanted the picture frame to be brown, I outlined with chocolate buttercream icing.
Then fill in your design using a tip like #4 or #5. Be sure to really fill in every corner since you want your transfer to be solid. You can smooth it with a spatula.
Once you've completely filled in your design, fill another decorating bag with icing the same color as the background of your cake. My cake was going to have a white background, so I used white icing. Outline and fill the entire design with your background color.
Place your sheet in the freezer for at least ninety minutes or overnight.
After icing your cake, pull the transfer out of the freezer. Quickly, flip it onto your cake and peel of the waxed paper. Finish decorating your cake with borders, lettering, or whatever else you desire.