Keep Exposure Constant
Keeping your exposure constant throughout the series of images is important and is best-achieved using Manual Exposure mode.
Keep the focal length and focus point constant throughout the series of images so that you can combine them later. Focus on something approximately a third into the scene and then switch the lens from Auto Focus to Manual Focus. Be careful not to zoom in or out while shooting your series of images or use a fixed focal length lens. Shooting with a longer lens is also recommended as wide angle lenses distort especially near the edges of the images and this can cause problems when you stitch.
According to Adobe, the images should overlap by at least 40%, but no more than 70%. Outside of this range, Photomerge may not be able to align the files correctly.