Using a PC or a mobile phone, which is the easiest way, you select your wanted wireless network during the setup of the adapter as shown in the guide below.
Guide for setting up the extender:
https://kb.netgear.com/24641/WN3000RPv2-initial-installationOnce that is done, connect the AmigaOne with an ethernet cable to the adapter. You can then run the "Connect to Internet" wizard on AmigaOS 4.1, choose Ethernet and auto for configuration, and it should be ready to use.

Afterwards, the computer will treat it as a normal cabled connection and connect on startup when you boot AmigaOS.