Save 10 villages in Endless
In v0.752a, the requirement is 10 saved villages overall in Endless. Wins can accumulate across runs. Check the total number of villages saved in Endless.
Open the Deckbuilding menu
From the mode selection screen, choose More Modes, then Deckbuilding. Once the card is unlocked, select it to start a run and choose your opening cards. The six villages shown on the Deckbuilding card are the target for completing one Deckbuilding run.
The exact fresh-save requirement on newer Steam Demo builds remains unconfirmed.