If you’re good at colouring in you’ll be great at this. Copy a favourite colouring page or print one from Google. Trim your coloring page, and tape it to the back side of a flat piece of glass. Use glass from a spare picture frame or similar.

Tape a another piece of wax paper to the front of the glass. Now you have a coloring page/glass/wax paper sandwich.

Begin colouring in your design with your coloured icing and place in the finished design into the freezer until set.

Un-tape the wax paper/icing, and quickly but carefully, up-end your design onto the cake. Press it down a little so it sticks.
Then, also quickly but carefully, before the design softens, peel back the wax paper to reveal the design.