Oracle Text

Creatures can't attack you unless their controller pays 2 for each creature they control that's attacking you.

Card Rulings

10/4/2004 The payment is made during the declare attackers step at the same time you are declaring the attacker.
10/4/2004 If there is more than one Propaganda on the battlefield, the cost is cumulative.
10/4/2004 If there are multiple attacks in a turn, then you have to pay for each attack.
10/4/2004 Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.
2/1/2007 In the Two-Headed Giant format, you still only have to pay once per creature.
2/1/2014 Unless some effect explicitly says otherwise, a creature that can’t attack you can still attack a planeswalker you control.