How are new conventions established in Contract Bridge?
Subject to certain constraints set by the governing authority (the A.C.B.L. in North America) mostly to restrict agreements intended primarily to interfere with opponents bidding, a bridge pair is allowed to make any agreement about the meaning of calls (including bids as well as passes, doubles and redoubles) and cards played as long as they present their system (in written form) to their opponents. During the bidding and play, whoever's turn it is can ask the one opponent what the other opponent meant with a bid or a card played. During the bidding, players must alert or announce certain of their partners calls if it has an (unusual) conventional meaning.

If a certain call is used between partners as having a certain meaning, the pair can be penalised if that call is not in their system description and/or not alerted / properly explained.

Note that all information exchanged between partners MUST be conveyed using the calls and the cards played (in the proper way). The cheats as recently discovered in top Bridge are usually using a different method: coughs, a certain positioning of the bidding cards, etc...

Here is an example of the form provided in North America by the A.C.B.L. for summarizing system agreements: 


In extended team play the accompanying system notes for a long-established pair such as Eric Rodwell &amp; Jeff Meckstroth can run to several hundred pages.