Firstly Bridging != WDS but I think you got that.

I have 2 WRT54Gs at home doing exactly this.

The Airport requires nothing to be changed. It just runs as a normal access point as is, as normal.

On the WRT54G you just need to put it in bridged mode and the associate it with the Airport just like you would a laptop or PC with a wireless card. Look under wireless and select the mode to be "Client Bridge". Then the security settings etc.

Then just plug something ethernet based in the WRT54Gs switch ports and that device magically becomes wireless.

Piece of cake. Let me know if you still need help might be easier on IM or something.