You can extend both wireless and wired, through wired is much better for performance. It is very simple just connect the express to your extreme via ethernet cable (make sure that both are powered on) and it is best if you reset your express to factory status since the process will do the needed for you.
Once the express is reset and connected via ethernet cable to extreme open airport utility and power on the express the airport utility will detect the express and will ask you which configuration you wish to proceed with (first choice would be extend your current extreme's click next and you are nearly done few moments (depending on your system configuration) airport utility will prompt you that the process is done and that the express will extend your extreme wireless network via ethernet
Thats it now you may disconnect the express and place it in any room that you dont have any or good wireless signal just make sure that you connect it to ethernet plug that is connected to any LAN port on your extreme.
One more tip you may use a LAN hub between the extreme and the express if you wish to use more LAN ethernet ports it will not effect the process. Did it myself and the whole setup is amazing.