Use a utility knife to cut off the bottom of the basket. Cut it along the perforations where the plastic is thinner than the solid base. Measure the height of the basket from underneath the rim and then cut the black poster board to that size.
Cut the white poster board into 4-inch strips. Cut each strip along the width to make equal pieces. Roll each piece into a tube and staple the ends to secure. Lay the paper tube on top of a piece of batting and roll the batting around the paper.
Cut the taffeta fabric lengthwise in half. Glue along the inner rim of basket. For a flowy, frothy look, glue bunches of the fabric to the same spot and be sure to cover the entire rim loosely. Tape or glue the marshmallows onto the fabric and the basket.