This isn't easy.
This is a reinstall; there's no fallback or downgrade here.
Then boot the installer DVD or a retail DVD that's at least as new as the target MacBook Air requires or newer, wipe the disk, reparition, and install Snow Leopard.
Or boot the installer DVD, wipe the disk, repartition, and install from your pre-upgrade backup if available.
I don't know that reloading your user files onto the old release will be particularly reliable here, as the preferences files and related baggage can and probably will have changes from newer releases that may or may not work on Snow Leopard. (This is where things get gnarly.)