Look at player, press "Trade" button ("v" by default), and request will be sent. Second player should do the same, to accept requset.
Also, you can use /trade 'player_name' command
Config file (config/trade.properties):
trade_swap_time_second - time to wait before items sawpping
request_time_second - request lifetime
unlimited_trade - allows to trade on any distance, from different dimensions
If you have issues/ideas: