You simply install it on each device, add some folders on each device, then pair a folder on one device with a folder on the other device and choose how the files should be copied (only one direction, the other direction, or both) and if you want to also sync deletions.

Syncthing is not actually the hard part, the hard part is how the two devices can talk to each other over the internet. I've set mine up to only talk over Tailscale (by using their Tailscale names in Syncthing).