The three main signals for defenders are:
Count, Attitude and Suit Preference. For a basic summary of these methods, click here.
When filling out the convention card, it is assumed you know about these signals; all you have to indicate (and decide with partner) is which way you are playing them. "Standard American" is:
COUNT: High-low to show an even number.
ATTITUDE: High shows you like it -- although the reverse of this is true in many parts of the world.
SUIT PREFERENCE: High shows the highest-ranking side suit.
You can give only one of these signals at a time, but you should mark whether you play these signals "standard" or upside down.
For count or suit preference, "standard" or "upside down" doesn't make much difference. For attitude, "upside down" is actually a little better, but not worth switching to if it will give you a headache. Standard is good enough for Bob Hamman, so it should be good enough for you.
In general, a discard should be an attitude signal (the first one, anyway). Subsequent discards can be used to show current count. The signal to partner's lead of a suit also tends to be attitude in most signaling systems.
Roman (or odd-even) discards/signals: This is a good method (but again, not worth a headache) whereby an odd card is encouraging. An even card is discouraging, and also suit preference (low for low side suit, high for high side suit).
Smith Echo: This signaling method (usually used only against notrump) is quite popular amongst experts. Playing hi-lo in the first suit declarer plays says nothing about that suit. It sends a message about the suit the defense has led at trick one. Hi-lo means "I like the opening lead suit." Lo-hi means "I don't like it." This signal is used by both defenders and requires lots of judgment and inference. If count is important (for example, a long entryless suit is in dummy), then that overrides the Smith signal.
This concludes our 26-part journey through the convention card. For anything that was not mentioned, try SEARCH on this same website (www.larryco.com).
Larry's Audio Tour of the Convention Card