That's it for the initial sketch! From this point on, press harder with your pencil to get a more defined sketch.
Step 6: Draw the rattlesnake's eye as a small circle inside the top arc near the top edge. Add a line above the eye for the outer structure. Add a tiny circle on the side to represent glare and a slit in the middle for the pupil. Shade the inside of the eye using a value that's lighter than the pupil. Add the corners of the snake's eye on either side using a dark value.
Step 7: Draw a couple of curved lines near the front of the top arc for the nostril. Draw a small triangle-like shape between the rattlesnake's nostril and the eye for the pit organ that senses infrared thermal radiation.
Step 8: Use the initial arc and circle as guides to draw the top part of the snake's head. Add a curved line above the eye for the bumpy structure. Follow the basic path of the guides as you darken the lines and create the shape of the rattlesnake's head. The line for the mouth should extend inside toward the middle of the initial circle.
Step 9: Draw the rattlesnake's gums, fangs and teeth at the top of the mouth. The gums should be a curved line just below the mouth line. Draw the two fangs as curved, pointy triangle-like shapes up at the front. You can make yours longer if you'd like. Add some smaller teeth near the back as well.