Read our article on 'How many Free Trades Do I Have?' to find out how many you have remaining.
In order to use your free trades, you merely need to place an order and have it filled.
Brokerage is charged when your order is filled, not when you submit the order. The first order to be filled will use the free trades.
Example: if you have 3 free trades and 10 trades in the market, regardless of when you placed the order, it's the orders that are filled first that will use your free trades. The remaining 7 orders will be charged $9.50 when filled.
If an order does not fill at all, the brokerage will not be charged.