Cropping people and cropping landscape images are a bit different. With landscape images you usually just crop until you have eliminated the distracting elements while making sure the horizon line is level. With people, you do the same with respect to removing distracting elements but you must ensure the subject is the primary focus of the frame. There are some things that you might have heard like "I have to shoot loose to leave space to fit an 8" x 10" frame" Or Cropping in post is for farmers. Or maybe it's fill the frame. Don't get caught up in the words, just remember to think before you shoot. Try to remove distracting elements in the viewfinder and frame the subject based on the intended final use of the image.