Order fabric with our clip art from Spoonflower; it's sold by the yard.
Determine the final height and width you would like for your apron to fit your particular flower girl. Add 1 1/2 inches to the height (for a 1-inch hem at the bottom and a 1/2-inch seam allowance at the top); double the width. Hem the sides, using a 1/2-inch or smaller hem, and then hem the bottom.
Measure your flower girl's waist, then add 36 inches (for 18-inch ties). Cut two lengths of 1-inch-wide grosgrain ribbon to that dimension. Use straight pins to mark the final apron width, centered, on one of the ribbons.
Sew a long gathering stitch in the top edge of the apron, and gather the apron so that it is as wide as the final dimension (hold it against the ribbon to check). Then pin it on top of the ribbon, centering the raw edge (you'll have a 1/2-inch seam allowance between the raw edge of the apron and the bottom edge of the ribbon). Distribute the gathers evenly. Pin in place, then sew down the center of the seam allowance. Pin the second ribbon over the first, and stitch them together along top and bottom edges. Hem ends and bottom.
Or download our larger version of the clip art, and print onto iron-on transfer paper. Cut out, cutting close to images, then iron the images onto an existing child-size apron.