how to extend an airport extreme using an airport express

Unfortunately, you don't indicate whether you will be using a Mac, a PC, or an iPhone/iPad to set up the Airport Express, so it would really help us provide accurate information for your particular case if you could please provide a few more details.

If you will be using a Mac, we assume that it is running a current or recent version of the operating system, correct?

We don't know what model of AirPort Extreme that you have as well, but will assume that it is a current or recent model, and it is working correctly at the present time, correct?

We also don't know what model of Airport Express that you have. Should we assume that you have the current version of the product....the model with two Ethernet ports?

If your AirPort Express has only one Ethernet port, some of these models are not compatible with current and recent Mac operating systems as well as the iPhone and iPad, so we may need to ask more questions if your AirPort Express has only one Ethernet port.