As mentioned above, many of the standard screenplay formatting software, like Movie Magic Screenwriter or Final Draft, include a stage play formatting feature. If you don't have the (about) $250 to purchase the software, manual formatting won't be too daunting a task.
Read through copies of published stage plays, preferably those of a genre similar to yours. Look for writing craft, specific stage direction jargon, and how the playwright uses scene descriptions.
Unlike screenplays, a stage play begins with a type of table of contents; first a character list with a brief description of each character (listed in order of importance), and a list of setting descriptions. The length of each setting description depends on how many fantasy elements are included in the script.
Begin to write your play. Keep a copy of your favorite stage play on hand as an immediate formatting reference, mostly for scene descriptions and dialogue.