Summarize:

Cut off the sleeves. For a fuller, fluffier look, line the inside with the batting from a pillow or old couch cushion, hot gluing it in place at the neck and armholes. To prepare for cold weather, wear a long-sleeve white shirt and leggings underneath the sweatshirt, or leave the sleeves attached and role them at the wrist if needed. Cut off a section of marabou boa, bunch it up and glue it to the back of the costume for a tail. If you don't have a boa, clump together some cotton balls or glue on a larger pom-pom. Glue the rest of your boa onto the front of the costume for a cute, fuzzy look. To add a pop of color, cut a large oval out of pink felt and attach to the front of the costume with glue or a sewing machine. Turn the cut-off sleeve inside out and glue it closed at one end. Snip this end off of the sleeve and fit it to your head, trimming more if necessary. With the other sleeve, cut out two triangles, about 7 in (2.5 cm) long each. Cut out two smaller, teardrop-shaped pieces of pink felt to glue to the ears. Glue the triangles onto the hat for floppy bunny ears. To complete the costume, cut out a carrot-shaped piece of cardboard and cover it with orange construction paper. Add some chopped-up green pipe cleaners to the top for a stem. You could use a real carrot for a shorter party. For example, a real carrot would probably be OK to last through an Easter party, but not a full day of school or a long night of trick-or-treating.
Use an old white sweatshirt for the main section. Use a boa or cotton balls for a tail. Make a chest piece with felt or boa. Make a bunny hat out of one sleeve. Make a fake carrot as an accessory.