To execute an after-hours trade, you log in to your brokerage account and select the stock you want to buy. You then place a limit order similar to how you'd place a limit order during a normal trading session. Your broker may charge extra fees for after-hours trading, but many don't (be sure to check).
Your broker then sends your order to the ECN it uses for after-hours trading. The ECN attempts to match your order to a corresponding buy or sell order on the network.
Example of after-hours trading
You might want to make an after-hours trade on a stock when it releases significant news after the market closes.
Let's say Apple (AAPL -1.64%) reported its quarterly earnings after the market closed for the day. The market initially read the report as negative. However, you think it's overreacting, and you believe the long-term prospects for Apple remain strong.
Log into your brokerage account and place a limit order to buy 100 Apple shares at $180 each. The broker will send that order to its ECN, where it will look for an order or combination of orders to sell at least 100 Apple shares at $180 or less. If it can match your order, the trade is executed, and settlement times are the same as during regular sessions.