How to Perform the Cable Crossover
The pectoralis major, also known as the chest, pectorals, or pecs, is made up of two heads - the clavicular head, also known as the upper chest or upper pectorals, and the sternal head, also referred to as the chest or lower pectorals. The chest is primarily built through pressing movements and movements requiring the arm to move across the midline of the upper body, across the chest.
The cable crossover is an isolation push movement targeting the sternal head of the pectoralis major and involving the movement of the arms horizontally across the midline of the chest. The clavicular head of the pectoralis major, pectoralis minor, rhomboids (middle back), levator scapulae (rear neck), anterior or front deltoids, and latissimus dorsi act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group(s) during the movement. The biceps brachii (comprised of the long and short heads), brachialis (lower bicep), triceps brachii (comprised of the long, lateral, and medial heads), wrist flexors, rectus abdominis, obliques, and erector spinae (muscle running along both sides of the vertebral column) act as stabilizers during this exercise. 
Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving.  The cable crossover is a brutal exercise to isolate, pump, and grow the pecs.
MTS Nutrition CEO Marc Lobliner demonstrates proper cable crossover form.
How to Perform the Cable Crossover
Begin by approaching the cable tower, moving the pin heights on both sides of the tower to shoulder height, attaching the single handle pulley to each cable, and selecting the appropriate working weight. This exercise can be perform with the pulleys below, at, or above shoulder height.
This write-up will walk through the exercise with the pins and handles starting at shoulder height. don't use 10lbs on one side and 20lbs on the other side. Uneven loading won't improve your gains and will likely lead to an injury. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions.
Once you've selected the appropriate working weight, grasp the handles with a neutral/hammer grip (palms facing each other). Your right hand will be holding the handle originating from the right side of the tower and the left hand will be holding the handle originating from the leg side of the tower.
If the cable tower is relatively wide then you can walk over to one side, pick up the handle, and then walk to the other side and pick up the other handle. Before beginning the exercise you should be standing in the center of the cable tower so that the tension on each cable is about the same. For many cable towers, this means you're standing directly under the pull-up bar.
Take a stance in between hip and shoulder width; your feet can be staggered or in-line with each other. You can use a traditional grip (thumb wrapped around the fingers), hook grip (fingers wrapped around the thumb), or a false grip (thumb and fingers on the same side of the handle).
After setting your stance and grip, take a deep breath, brace your abdominals for impact and stand tall with an upright posture, slightly forward lean of the torso, and slightly flexed hips and knees. With the handles starting at shoulder height, your elbows straight (but not hyperextended) or slightly bent, and the shoulder slightly internally rotated so that your elbows are pointing towards the wall behind you, begin pushing the two handles together and towards each other.
Maintain the same elbow position and continue pulling both handles together until your pecs are squeezing together and the handles are almost touching. Some trainees prefer to and get a better contraction in the pecs with pushing the handles past each other, across the midline of the chest, so that the lower arms make an X-shape. Hold the cable handles in this position for 1 to 5 seconds.
Then in a controlled motion slowly allow the cable handles to return back to the starting position. Complete for the desired number of repetitions. Some lifters choose to exhale while pushing the handles together, at the top of each rep, or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
Throughout the entire movement, your torso angle should remain fixed, posture should remain upright, and your elbow position should remain fixed. If you find yourself swinging your body, raising your shoulders towards your ears, or unable to control the cables as they return back to the starting position then the weight is too heavy.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc...) but strive to improve every training session.
Cable Crossover Form Tips
Minimize Excessively Torso Lean - If you ask 10 different people about cable crossover form you're likely to receive 11 different answers. Everyone seems to have an opinion on the preferred cable height, stance, elbow position, and torso position.
Regardless of how you tweak those variables, ensure your spine stays neutral to ensure an upright posture. Spinal flexion into a crunch-like position typically leads to a form breakdown (excessively bent elbows and raised shoulders) which minimizes the stimulus of the target muscles.
Hold at the Top - If you're looking to increase intensity then experiment with holding the cable handles at the fully pushed together position for 5 to 10 seconds. Really focus on squeezing the pecs together - imagine some placed a pencil in between them and you're trying to pinch the pencil so that it doesn't slide down.
This will increase time under tension and the burn in the pectorals. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Avoid Momentum - The cable crossover provides maximum benefit when it's performed in a controlled full range-of-motion.
Check your ego at the door and don't attempt to cable crossover 150lbs per handle. Excessively heavy weights on this exercise will lead to body swinging in an effort to generate enough momentum to pull the cables together.
This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
References1) "Cable Standing Fly." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 3 Nov. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 3 Nov. 2015.