The Roaming Network is far better, and you should really be using it. If a cable isn't an option use Powerline adapters.
However, if you still cannot use this for some reason, then you need to use AirPort Utility on your PC (or iPad) to set the Extreme to "extend a wireless network" when you set it up. It's as simple as that.
A Powerline adapter is an adapter that allows for wired networking along the power lines of your house, making Roaming an option (as you don't have to run cable). Although, they don't always work (but do mostly) so make sure you understand the store's return policy.
(Sorry I couldn't write a guide myself but I have some other stuff I need to get done).
Try the follow method: Configuring two wireless routers using 1 SSID & doesn't have to be the same brand routers.
Hope this helps