To make a short sell simply open a stock and click the Sell button. Making an order from there is the same as buying stocks. You can place a limit short sell order for any stock on 10n which is marked as shortable at any time and a market and stop order during the regular trading hours. You can only have one position of a stock at a time – owned shares or shorted shares.
For example: If you have 10 shares owned of ION, you must sell them first and then you are able to make a short sell. The opposite is in effect when you want to buy shares of a stock that you have shorted positions of.