Now and at the start of your next turn:
Each player chooses a Kingdom card to be Forbidden (put its randomizer on the mat).
At the start of your next turn, empty the mat.
When any player plays a Forbidden card, they gain a Curse.
There is a single play mat to hold the Forbidden cards, which is shared by all players. When any player selects a card to be Forbidden, put its blue backed randomizer card onto this mat to indicate this. The cards are placed face-up for all to see.
Each player must choose a card to be Forbidden, starting with the one who played the Treaty. The card chosen does not need to be a Kingdom card from the Supply. (Picking an unused card would usually do nothing — except in cases such as when playing with the Black Market.)
Only Kingdom cards may be chosen; standard Supply cards, such as Treasure, Victory or Curse; or cards such as the Prizes from Cornucopia cannot be Forbidden. Apart from that, there is no distinction made between card types — a Victory card may be Forbidden just as an Action or Treasure card could (although a Forbidden Victory card is unlikely to trigger the penalty, unless it is also an Action or a Treasure).
Players may choose a card which is already Forbidden, which has no additional effect, the Curse penalty remains the same.
Treaty itself may be chosen as a Forbidden card, but this will only affect copies which are played after the current Treaty.
Playing any copy of a card on the Forbidden mat causes the player doing so to gain a Curse each time they play the card.
For example, if any player plays a (Forbidden) Village multiple times, they must gain a Curse each time. If they then played a Witch (which was also Forbidden) they would gain an additional Curse.
A Forbidden Reaction would cause the penalty when revealed, just as an Action would when played on the table.
You gain a Buy and 1 Coin this turn and next turn.
This card is a Duration card and will remain in play until the Clean-up your next turn, when it will empty the 'Forbidden' mat of all cards. It doesn't matter if more cards have been Forbidden with other copies of Treaty, the entire mat will still be cleared.
The 'Forbidden' mat contents are independent of any Treaty cards. If a Treaty card is removed from play before it has a chance to clear the Forbidden mat, then those cards will remain Forbidden until such time as another Treaty clears the mat.