Here are a few steps to help you trade on commodity futures with an online stockbroker.
Deposit the margin amount to the broker.
The broker will place the order on your behalf.
Matching of buyers and sellers happens on the online exchange platform.
The exchange will determine the settlement price for the different commodities when the market closes.
The difference is either debited from or credited to your account based on the price movement.
You have to pay the margin shortfall if it exists.
You have to close your position before the contract expires.