The set-up is the same as the two-player game, except that the automa opponent doesn't use any tokens. You take all the turns. When you roll a 10 you collect tokens on your three crowns as usual but the automa opponent doesn't.
When you sell a card to gain more tokens, instead of discarding it, you play it as a developed property on your opponent's side. You can choose which district to play it on, but it must match suit as in the regular game. You'll want to put most of the cards in one district and write that one off as a loss, but sometimes the suits won't let you.
If you want to sell a card that can't be placed with either suit you may discard it.
You only run through the deck once since there are no (or very few) discards. Scoring is the same as usual.
To make the game harder:
If one of the suits on your discard is one of your automa opponent's crown suits, you must place it using that suit if possible. If not, you can place it using the other suit. This mimics the two-player game where your opponent is more likely to be able play cards with suits from their crown cards. It makes the game more difficult because it limits where you can play the opponent's cards.