To short sell a product on your Active or Trader account, you simply place a sell order in a product you do not currently own.
Short selling is restricted on certain products, such as U.S. securities and category D products. In this case, if you try to take a short position, you will recieve an error message.
It is not possible to hold both a short and a long position in the same product. Placing a sell order in a product in which you currently hold a long position will sell your existing shares.