Please HELP!!! Setting up airport express

Hi all,

I'm new to the community and totally stuck! LOL :-(

Toys I have: draytek 2820n wireless router

apple airport express (white one that looks like apple TV)

iPhone 4

Hifi system I have connected to the airport.

network switch plugged into draytek 2820n

iTunes on computer (windsows 7) - connected to router via net switch

All iTunes media on external NAS drive - connected to router via net switch

My home network is all wired via the gigabit net switch.

I would like to be able to use my existing wifi network to airplay music from my iPhone and my gigabit wired network to stream from iTunes on my computer.

I do not want to add another wifi network to my homem network. The draytek 2820n router and the airport are next to each other and the hifi system.

Having read the setup guide for the airport express I'm left baffled! ALL of my ethernet devices are connected to my network switch. This in turn is conected by ethernet cable to the gigabit port on my draytek 2820n.

What I don't understand is how to connect the airport to my system. As the router, airport and hifi are next to each other, I would like to connect the airport by ethernet cable directly to my router. Then I would like to disble wifi on the airport and use the wifi network provided by my router (draytek 2820n) to stream music to my system.

Is this possible?? and if so, could someone please tell me if I use the LAN or WAN port on the router and the WAN or LAN port on the airport express?

Any help would be fantastic as having read the setup guide, I do not understand if it's bridge, extended etc?

Many heartfelt thanks in advance for any help offered as I'm lost!