The first step is to make sure you have all the requirements for system link play. First of all the game in which you intend to play must support system link. To confirm it's system link ready. Look on the box and look for the system link icon. The game box or the manual itself will also list the maximum amount of supported players for the game.
Once you have verified that the game you intend to play over LAN supports system link. Proceed to step 2.
2.Turn off the power and unplug both Xbox 360's
3.Connect each Xbox 360 to a separate TV or monitor.
4.Connect an Ethernet cable to the Ethernet port on the back of each Xbox 360 console.
Your all set. Plug the power back in on both consoles and turn them both on and start playing. Refer to the games instruction manual for any game specific settings