I find it easier to do on grayscale pictures. Like Cornflake said, You just draw it on, but instead of taking the background color and trying to cover up the rest of the ear, just leave it be. On renders with a grayscale background, its a lot less noticeable because your attention is mainly focused on the color of the render in the foreground, instead of the lack of color in the background

All you have to do is, instead of following the pointed ears, just draw them like that. On renders,you don't have to follow very line! When you've made the lines just take whatever color the background is and make the part of the ear that your getting rid of and covor it up with the background color. That's how I do it anyway,you may have another way of rendering that my tecnique won't work for.