Once there, you can start a new trade (only if you have a trading pass, which you must buy to be the one starting a trade).
When you are in the trading page, just click items you want to trade (or add money). When you are ready, enter your password in and click confirm.
After that, the other person needs to add their stuff the same way, or just confirm the trade.
The final step is the trade-starter confirming it one more time. Once that is finished, the trade is complete~