simple. I’m not sure that all are the same but this is how I did mine.
1. Remove the cover of an existing phone line and look at the configuration of wires. Ours has 3 sets of two wires – (3 different colors) 2 sets are connected, one is not.
2. go to the phone line that you want to convert and change the configuration to the same as the existing phone line.
On ours it is connected with screws and is fairly simple to change.