A good portrait has five key elements: location, lighting, composition, emotion, and technical settings. A great portrait is generated when all five of these elements are perfectly executed. The quality of the portrait decreases if any of these factors fails. But another thing would be correct camera settings, compositions, angles, lighting, backdrops, and poses which are all useful just like you have to focus on the subject, finding the right location, made sure their poses are right, capture emotions and expressions, and importantly understand how shutter speed and aperture work. But some bad things are composition, distracting background, subject too close to the background, not enough focus, slow shutter speed, poor lighting, and bad timing so those are things are can help you with taking portraits.