1. SIGN-GIVING STANCE
When giving signs, the catcher's job is to allow only the pitcher (and middle infielders) to see the signs. To do so, the catcher will place the glove on the outside of the left knee. This blocks the third base coach from stealing signs. The sign-giving stance should be very relaxed, with the catcher sitting on his toes and the knees kept in tight. Young catchers have a tendency to open their legs up too wide, enabling opposing players and coaches to see the signs being given. The forearm and wrist of the bare hand should be sitting gently on the thigh, positioned deep, up against the protective cup. To avoid having pitches picked, a catcher should avoid giving signs too high or too low.
2. RECEIVING STANCE
After the sign is given, the next step is to get into either the receiving stance or the blocking/throwing stance. With nobody on base, and less than two strikes, the catcher will be in the receiving stance. This stance allows the catcher to concentrate exclusively on catching the baseball. There is no blocking or throwing needed while catcher is in this stance.
There are many different types of receiving stances. For catchers ages 12+ (depending on strength of catcher) I teach them to receive with their feet a little wider than shoulder with apart. With their toes pointed slightly out, and the front of their left foot even with the instep of the right, I have them roll onto the inside of their feet (with most of the weight transferring to the front of the left foot) as they get down into the stance. This stance is going to feel awkward and probably hurt a little bit when first learning it, but it allows the catcher to "drop-knee," which will be discussed later.
In the receiving stance, the catcher's back is almost completely straight. The glove should rest comfortably in front of the lower chest - not too far into the catcher's chest protector, yet not too far away from the body that the catcher extends his arm. The catcher's glove-hand wrist should rest gently on the thigh, a few inches before the knee. The elbow should be bent with no tension - Leave it nice and loose. The throwing or bare hand should always be behind the back. Because there is no threat of a baserunner advancing or scoring, a catcher may attempt to scoop or pick a ball is in the dirt no blocking required!
3. BLOCKING/THROWING STANCE
If a catcher is forced to use this stance, his responsibilities increase dramatically. The catcher must still receive, but also must block any pitchers in the dirt and throw runners out attempting to steal. While in the blocking stance, the catcher must raise his tail up a bit to be able to block pitches better, shift quickly, and throw to second base with more ease and accuracy. To do this, the catcher's feet should be a little wider than they were in the receiving stance. Once again, the catcher will roll his feet in, but not as much as in the receiving stance. The catcher still wants to be on the balls of his feet, without sitting back too far. In an effort to avoid blocking the umpire's vision, do not allow the catcher to get too high while in this stance.
While in this stance, the bare hand sits behind the glove instead of behind the back. This allows the catcher to either... quickly grab the ball when moving to throw out a runner, or keep behind the glove when going down to block. Both wrists should be resting on the catcher's thighs (or close to the thighs). It's very important that the catcher does not stand flat footed while in this stance as this makes blocking and throwing more difficult.
For more information, check out my book Coaching the Beginning Pitcher.