You buy an ETF the same way that you buy a stock. When the stock market is open, you enter an order with your broker at either a market or limit price and pay the appropriate commission.
Because ETFs are designed to track predetermined indices, their holdings and asset values are easily determined. Most ETFs calculate and post the underlying asset value of their shares every 15 seconds during the trading day. This compares to mutual funds disclosing their holdings only 4 times a year with information that is up to 60 days old.