I am putting afondant bow on the side of a square cake. The bow will be position in the middle.

Any suggestion to keep the bow from falling off or securing it to the cake?

Will your cake be covered in fondant? A light brushing of water will attach fondant decorations to a fondant covered cake. But if the bow is larger and because it is on the side of the cake, I would either use royal icing or melted white chocolate.

Will the cake be iced in buttercream or fondant? If buttercream, I would use a toothpick to try to secure it, but if it's fondant then you can use water to "glue" the bow to the cake...

I'd use royal icing and secure it with a toothpick until it dries.

The cake will be decorated in fondant.