Q: I have Airport Extreme How to install wifi extender

Unfortunately, there is not enough information in your post to be able to answer your question.

Will you be using an AirPort Express, an AirPort Time Capsule, or another AirPort Extreme to extend your existing AirPort Extreme network?

How will the extending AirPort connect to the "main" AirPort Extreme?  Will it connect using a permanent, wired Ethernet cable connection (highly recommended for best performance).....or.....will the extending AirPort connect using wireless?  (which will result in a loss of about half of the potential speed on the network, so you would want to avoid this type of setup, if possible)

Finally, what device will you be using to set up and configure the extending AirPort device?  If you will be using a Mac, we need to know what operating system it is running.  If you are not sure about that, click the Apple icon in the upper left corner of the screen, then click on About This Mac.  Post back with the OS X Version number that you see there.

Or, will you be using an iPhone or iPad to set up the new extending AirPort?