So - for those who don't use Facebook, it's not that easy to find information about growing terrestrial orchid seed on cardboard by searching.
Subrosa has provided a brief description above. Another is in this thread:
Cardboard germination of Calopogon tuberosus
There aren't any more detailed descriptions I've found here on Orchid Board.
In another forum OB member naoki mentions a book about growing Japanese orchids this way, but it is in Japanese.
The most detailed discussion I found is in the
Scottish Rock Garden Garden Club Forum. The medium used is quite complex, and I suspect that isn't necessary.
Here and there you can find photos of people's cardboard setups without much explanation.
Some people describe shredding the cardboard, soaking in pure water for days to a couple of weeks, then mixing it with the rest of the medium. Other people stand wet cardboard pieces vertically, separated by thin vertical layers of soil mixture. The seeds are sprinkled over the whole thing. Then it's dampened with rain water, enclosed to keep humidity high, and put in a cool dark place (temperature appropriate to the plant) for days to a couple of weeks.
I haven't done it yet, but I figured I'd share what I found.