If you are running a different operating system, you can use any search engine to look up "Find local IP address on <your operating system>" to typically find tons of guides for any platform.
It is recommended to set your host machine's IP address to be statically assigned instead of dynamic/DHCP. This ensures that the IP address will not change through machine or network reboots.
Check What Port is Being Used
If using the default installation, Sonarr runs on port 8989. In most cases, this port is not changed and does not need to be changed.
The simplest way to check is to go to Sonarr's web GUI, go to Settings -> General and note the value entered into "Port Number".
Ensure Sonarr is Accessible Across Your Network
To ensure that Sonarr is accessible across your local network, check the following:
In Sonarr's web GUI, go to Settings -> General and enable advanced settings. Ensure that the "Bind Address" is set to *, as this makes Sonarr bind to all network interfaces on the host machine.
Check any enabled firewalls to confirm that the port running Sonarr is not being blocked.
(Windows): Ensure that Sonarr has been run as administrator at least once.
Check If You Are Using a URL Base
In Sonarr's web GUI, go to Settings -> General and check the value of "URL Base". If you have nothing set, you can move on. If you do have a value set, please remember the set value as it will be necessary when setting the host within LunaSea.
Connecting in LunaSea
The host is a combination of multiple values found above:
The local IP address
If being used, the URL base
Combine all the values into the following format: <IP address>:<port>/<URL base>
For example, if Sonarr is running on port 8989 on a machine that has the IP address 192.168.100.100, the host is: http://192.168.100.100:8989.
The API key is copied from Sonarr's web GUI, by going to Settings -> General and finding the API key value.
Custom headers allows users to attach custom request headers to each API call that is made. This is typically an advanced feature, and is not necessary in most network configurations.