If you plan to build a LoRaWAN relay, the Relay branch of LoRa Basic Modem is the starting point. Of course, it is important to read the LoRaWAN Relay standard.
Compared to the common LoRaWAN End-device, there are modified behaviors on the device if they should support a relay. It will have an added Wake-on-Radio (WOR) frame, enable CAD, etc.