1/ Connect your spare SH by Ethernet to a PC/Laptop. Best not to be online to the other SH when doing this. In the background, the spare SH will be crying out via the arrow light for its DOCSIS connection. Ignore that.
2/ Go into the spare SH GUI settings (192.168.0.1) and turn off all Firewall functions.
3/ In the GUI, set up your wireless SSIDs and behaviours as you would want them to be. Consider keeping a common WiFi password across the wireless estate.
4/ In the GUI, turn DHCP OFF. Set the LAN IP address of the spare SH to be 192.168.0.254. IIRC, there is a quirk whereby you need to set the ending IP address as well to something silly like 192.168.0.253. Comeback to us if there is a difficulty when you do this. Apply the changes. While the spare SH is faffing with the changes, your connection to it will be disrupted and the spare SH will still be frantically looking for DOCSIS! When that settles down, switch the spare SH off and disconnect from it.
5/ Now connect your laptop/pc into the main SH (which is still connected to the Internet) and set the ending IP address to 192.168.0.253 (if not already so). Apply the setting.
6/ Connect a LAN port on the spare SH to a LAN port on the main SH. You can do this by direct Ethernet cable or Powerline if the AP is going into another room.
7/ Switch the spare SH on and it should work straight away. It will be GUI accessible via 192.168.0.254.