Signals are used only on defense.  Do not Signal Dummy. And Dummy does not Signal Declarer.  (This would be a major impropriety.) And Signals are done only with the cards. Gestures, grimaces, moss code, winks, kicking, and cell phones are not to be used. Let the cards do the talking. 
      There are 3 basic Signals in Bridge.
Attitude /  Count /   Suit Preference
In order to avoid confusion, and there might be some when you first use signals, (and maybe even later), let the Attitude and Count be a preference over Suit Preference. Actually Attitude should count more than the Count. Miss use of Signals
can be the cause disastrous defensive errors and can lead to partnership break up, divorce, bodily injury and possibly even death. But do not be discouraged; the rewards are great. Still make sure partner is not armed.

Suit Preference
This is probably the most complex and abused of the 3 Signals. Does this mean we should not use the Suit Preference Signal.? NO! After all, cars are complex; does that mean we should ride a horse. (Which is also complex in its own way.) Some people say that complexity and abuse are what make Bridge a great game. 
      But enough of this. Let's see how Suit Preference works. 

t  used primarily against a suit contract.
t  usually only two suits are Preferenced. 
t  the suits are not trump and not the suit being led.
t  that leaves 2 suits. The higher ranking suit is 
     preferenced  by playing or leading a high card; and
     the lower ranking suit is preferenced by playing or
     leading  a  low card.
play or lead Low Card
play or lead High Card

If you want your Partner to return the Lower ranking of the two Preferenced suits, then play a low card. If you want your partner to return the Higher ranking of the two Preferenced suits, then play a high card. Do not worry if Declarer knows this information. Your partner will make better use of this information than the Declarer. 
      More important is to have a Partner who can see and read the Signals.  This is what makes for an efficient and effective defensive team.
      In EXAMPLE 1. below you are sitting West and lead the A C against a 4 Spade contract. Your Partner sitting East plays the 8 C. This can not be an Attitude Signal since the Q C is sitting in Dummy. So the 8 C is the start of a high/low signal showing an even number. In this case hopefully a Doubleton. (A Singleton is also good.) You continue with the K C and your Partner completes the signal by playing 2 C. Declarer follows suit. If Declarer started with 4 Clubs you want your Partner to trump Clubs two times. But if your Partner trumps the Club at trick three, how does he know how to get back to your hand? Is it with a Diamond or a Heart. Here is where the Suit Preference Signal comes in. 

1.    DUMMY
S A Q J 
H 10 9 
D K J 8 7 6 
C Q 9 7

S 9 6
H 6 5 4 3
D A  4 3
C A K10 3

S 5 4 2
H  J 8 7 2
D 9 5 2
C 8 2

Consider that the two Preferenced suits: not the suit being led (Clubs) and not Trump (Spades). That leaves Diamonds and Hearts. You obviously want your Partner to return a Diamond after he trumps the first Club. So lead the 3 C. This signal says: the 3 C is low, so return a Diamond. (The lowest of the two preferenced suits.) If as in EXAMPLE 2.  you had the A H, then play the 10 C at the third trick. This signal says: 10 C is high, so return a Heart.

WEST        2.
S 9 6 
H A 5 4 3
D 10 4 3 
C A K 10 3

In EXAMPLE 3. below your Partner leads the A S against a 4 Heart contract. Your Partner will not continue Spades and will make a switch to one of the two Preferenced suits: Clubs or Diamonds. (Not trump and not the suit being led.) But which one will your Partner choose? You can help him out with a Suit Preference Signal. 

3.      DUMMY
H A 9 8 7 6
D Q 10 9
C Q 8 5 4


S 10 8 2
H 9 6 
D A K J 3
C 7 6 3

 Play the 10 S on your Partner's lead of the A S. The high 10 says: "Partner lead the higher ranking of the two Preferenced suits which is Diamonds." If Partner sees and reads the signal the hand will most likely be set.
     If your Clubs and Diamonds were reversed; you had the 
A K J 3 of Clubs and the 7 6 3 if Diamonds you would signal with the 2 C. This says "Partner lead the lower ranking of the two Preferenced suits which is Clubs."