The description is full of some side notes about layer modes, and what you do for each step.

Step 1: It's literally just lineart

Step 2: You outline the area with the color you want to use, and use the bucket tool to fill in. You can also just color it in if you want.

Step 3: You can use fill bucket if you want for the simple shading. The dots around the pupil were made with watercolor brush.

Step 4: You use the airbrush for shading the iris of the eye, since the darker color shading wasn't already there. You erase the scelra shading with a soft eraser, because it wouldn't look that good with really dark shading there.

Step 5: Draw a short line with the watercolor brush at the bottom of the iris. Set the layer mode to add.

Step 6: Use the airbrush on a layer clipped to the lineart. Use a color a bit lighter than the one you used for the lineart, and color a bit of the edge of the eyelashes. But if you used black for the lineart, dont do a grey highlight there.