We first downloaded and installed the Nest App from Google Play.
To add the Nest Protect, we selected the settings gear icon on the top right of the Nest App's home screen, and then selected Add product.
We could then use the Nest App to take a picture of the 2D bar code printed at the back of the Nest Protect.
To continue, we first made sure we were on the same Wi-Fi network that the first Nest Protect had already been set up on.
Compared to setting up the first Nest Protect, setting up the second one was a whole lot easier.