It is no longer possible to obtain a witch skirt, or any other part of the witch costume. During the 2008 Hallowe'en event, players had to go trick-or-treating in Rimmington, Port Sarim, and Draynor Village to receive candy from certain NPCs. For this part of the costume, five wrapped candies needed to be received. Maggie would give you the item when you had all the wrapped candies.
Male characters cannot wear the witch skirt, or any other part of the witch costume. Male characters would receive a warlock costume, and if they try to put a witch skirt on (if they changed their gender), it will turn into warlock legs. Also, if a female character tries to wear a witch skirt while changing gender, it will also turn into warlock legs.