Choose a color to use as the key based on the foreground content. Not all chroma keys need to be blue or green, technically the chroma key is any color, but these colors see the most use. Set the editing equipment or software to extract any instances of the chosen color from the image it is given. Then place that image over another image, allowing the background to show through the holes left after removing the color. Generally blue looks best when placed against Caucasian skin tones. Ensure that no other instances of that color exist in the shot, as it is also extracted along with the background screen color.
Set the screen as far back from the foreground as possible. Any light reflected from the screen contains hints of this color. This spill of color can cause portions of other objects to disappear when the chroma key is applied. By keeping the screen far away from other objects, it eliminates any unintended color spill.
Use proper lighting on the screen. Variations in values, including changes in color or hue, which result from different levels of light or shadows, may cause some portions of the screen to remain while removing others. Avoid this uneven key by ensuring that the entire screen receives light levelly with dedicated lights. If you use the same lights on the screen as used on the foreground subjects, then the resulting shadows cause an uneven key.
Control camera movements as much as you can. If you have a camera movement on the foreground image, but not in the inserted background image, you shatter the illusion and draw attention to the effect. Replicate movement as close as possible between the composite images.