Step 15: Between the eyes, draw a couple of small, curved lines for the bridge of the nose. At the bottom, right on the edge of the initial circle, draw a short line for the base of Thanos' nose. On either side of that line, draw a short, slit-like line for the nostrils. Add a line on either side of the nostrils to finish the shape. The nose should be pretty small.
Step 16: Under the nose, draw a long, horizontal line for the mouth. Thanos' mouth should be near the middle of the initial arc. There should be a big space between the nose and the mouth. Draw a shorter line under the mouth for the lower lip. Starting on the sides of the nose, draw two lines that curve toward the tips of the mouth for creases. These lines should be long, and they should bend a bit near the middle.
Step 17: On the left side of the head, draw a curved line for the first ear. Inside the ear, draw a few short, angled lines for the inner structure. On the right side of the head, draw Thanos' other ear the same way.
Step 18: Use the remaining shapes as guides to draw the rest of the head. Simply darken the outer edges of the initial guides to create the shape of Thanos' head. Make the sides of the head a bit narrower than the initial circle as you darken the lines. Leave an open gap near the ears to indicate the ears connected to the head. At the bottom, make the lines more angular to define the jaw and chin.
Step 19: Add a few lines within the head for extra detail. Draw a series of diagonal lines on the sides of the eyes for the scar-like marks. Add a series of vertical lines on the chin for the creases found there. Don't make these lines too straight. Curve them and disconnect them a bit. The lines on Thanos' head seem to be scars even though they're very symmetrical. The lines on the chin seem to be naturally occurring.