Gently remove your pieces of cake from the plate (with a spatula, or your hands if you're adventurous) and place them on wax paper or directly onto a cookie sheet.
The two large outside pieces will be the snakes body. Flip one of them upside down again, and stick the two ends together to make an 'S' shape. You can use a bit of icing to stick them together if the cake will be moved around a bit.
Careful when moving your pieces, because your cake mix might be more crumbly than mine. Gluten free cake mixes are surprisingly sturdy!