That's a great question.
When it comes to placing the actual order, what will happen is they will give you an option to "sell short" (once you have margin approval).
For example, on my TD Ameritrade order screen, it displays:
- Buy
- Sell
- Buy to Cover
- Sell Short
Buy to cover is to what you use to buy back your shares and exit your short position.
Hope that helps and good luck with the shorting, now is the time to do it!
