Make the legs: Roll the white fondant into two 15-inch-long logs for the legs of the witches, using your hands or a bowl scraper to smooth the logs out. Use a ruler to measure out twelve 2 1/2-inch-long lengths and use a paring knife or pizza wheel to cut the lengths. Round one of the ends of each of the legs to attach the shoes to.
Make the stripes on the tights: Roll out the black fondant about 1/8-inch-thick with a small rolling pin. Cut even 1-inch strips, and then cut across to make 1/4-inch-wide strips. These will be the stripes on the witches' legs. Use a small amount of shortening to attach 3 strips to each of the legs. Flip the legs over and trim off the excess black fondant using a paring knife or pizza wheel, so the stripes wrap around perfectly.
For the shoes: Divide your red fondant into 12 even pieces, each about .1 ounce (about the size of a raspberry). Roll a piece of fondant into a ball to start, then form it into a teardrop shape. Make the tip come to a point with your fingertips and curl it back slightly. Place the curled tip of the shoe face up on your surface and use your fondant tool to press in and down slightly, making the heel of the shoe; hold the shoe on either side to help it keep its. Use a ball tool opposite the heel to make a little hole where the leg will attach. Repeat with remaining pieces of red fondant, making a total of 12 shoes. Coat the shoes in a light coating of shortening using a paintbrush. Dip the shoes into the red edible luster dust. Attach the legs with a bit water, pressing the rounded end of the leg into the top of the shoe.
Allow the legs and shoes to dry for at least 30 minutes. Place the legs in pairs on a platter, leaving a good amount of space between pairs. Press a cupcake upside down on top of each pair of legs and serve.