When drawing a dog, it is often easiest to start with basic outline of the head before you start on the ears. There are a few different ways to draw the dog: realistic, cartoonish or combination of both. There are also several different "styles" of dog's ears: upright, droopy or floppy. This tutorial will instruct you on the basics of how to draw each type.
Drawing a prick, upright ear
Draw a triangle shape on both sides of the circle at the top.
Blunt the tip of the triangle a little to soften it up.
Draw a slightly smaller triangle shape inside the one you just drew. This is the basic shape for the upright ear.
Drawing a droopy ear
Draw an teardrop shape on both sides of the circle with the smaller end at the top of the head.
Erase any lines inside the teardrop.
The teardrop shape can hang the length of the head for a long droopy ear or half the length for a short droopy ear. This is the basic shape for the droopy ear.
Drawing a floppy ear
Draw a triangle as you did for the prick ear.
Draw a smaller triangle inside the first triangle.
About halfway down the triangle, draw a line across the entire length and extending slightly past the edge. Erase what is above this line.
This will be the bottom for another, upside down and smaller triangle. At a slight angle away from the center, finish the upside down triangle. Erase any lines inside this triangle. This is the basic shape for the floppy ear.
Using these basic guidelines, you can make the ears as cartoonish or as realistic as as you want. Shading and softening lines will give a more realistic look. Consult pictures of dogs to use as a guide for shading.