When open a new position, a trader should transfer the margin token (Quote asset) to its account to ensure its margin is sufficient, that is
(AccountBalance + UnrealizedPnL) >= Position * MarkPrice * InitialMarginRatio.
With the available margin, a trader could go to "Trade” page to input the trading pairs, click on "Buy/Long” or "Sell/Short” button to create a trade.
To close a trade, just follow the reverse procedure – go to "Trade” page , select the contract you have traded, execute opposite position of existing trades, and margin plus your PnL would be released to your account. Alternatively, you could go to "Pool” page to view the list of trades you have and close the position.
Note that trader can only trade with a pair that have existing market makers, that is, the pair should have been created and provided liquidity by LP.