Slip a piece of aluminum foil in the center of the onesie underneath the entire freezer paper stencil. This will prevent the paint from getting onto the back of the onesie. Use a small amount of fabric paint on a sponge brush to carefully start applying paint to the onesie.
When the paint is dry, carefully peel away the freezer paper revealing the design. To add alternate color or embellishments, simply repeat the freezer paper process. For smaller embellishments, like the swan's bow tie, use a fine-point paintbrush to freehand it.