The default settings do work out of the box, with the default PSK.
However, you should connect your nodes once to one of your phones, for changing your channel settings (channel name, speed/reach settings…). When you do that, the default PSK will also change, and your privacy will be better.
At this stage, you can’t set the PSK manually, and therefore you have to set the setting with the APPs share function (otherwise you end up with devices with the same channel name, but different PSKs).
Once you have set up the first device, share your settings with the app, and set the settings to all nodes. When ready, just put the nodes wherever you like, and they will automatically act as repeaters for your private mesh.