There are several ways to do this but I choose to use funfetti cake mix Halloween edition and twizzlers (the fruity kind). Make your favorite cupcakes complete with your favorite icing. If you don't want Twizzlers you can use black licorice for the legs.
-Funfetti cake mix (Halloween edition)
*1 cup water
* 1/3 cup oil
Make you're favorite cupcakes as normal
When cool, frost the cupcakes. Use the grape flavored twizzlers for the spider legs.
Cut the twizzler into fourths with scissors. Then cut into thin strips. Top additional cupcakes with sprinkles.
Put the legs on a frosted cupcakes. Place an oreo on top of the twizzlers (you might want to spread additional icing on the oreo to ensure it sticks).