Start with a personality shape
Choose a body shape before choosing an expression. A circle feels soft and cheerful, a long wedge feels fast, a tall oval feels proud, and a low rectangle can feel stubborn. Push the choice far enough that the silhouette remains distinct without internal detail.
Draw three thumbnail silhouettes in a few seconds each. Do not add eyes. Pick the one that communicates the clearest attitude. This prevents the face from carrying all the character while the body remains generic.
Exaggerate one feature
Make one part noticeably different: an enormous fan tail, a tiny mouth, a high forehead, long trailing fins, or a very short body. Let nearby parts support that feature. A large tail works better with a compact body; long fins need open space around them.
Avoid enlarging everything. When every part competes, the design loses its hierarchy. One major exaggeration and one smaller echo are enough. For example, a broad tail can be echoed by a broad side fin while the eye and mouth remain simple.
Build expression from direction
Place the pupil toward the direction the fish is moving. A pupil near the front of the eye feels attentive; one near the back feels hesitant. Tilt the upper eyelid down for determination or curve it upward for a gentler expression.
Use the mouth sparingly. A short upward curve reads as friendly, a flat mark reads as calm, and a small open oval reads as surprised. Check the face at its final display size. Tiny expression lines that vanish do not help the design.
Simplify anatomy without breaking it
Keep the eye in front of the gill area, attach fins to clear roots, and taper the body into the tail. These relationships preserve the idea of a fish even when proportions are playful. You can omit scales and extra fins if they do not support the character.
Use overlap to show depth. A side fin crossing the body feels connected; a far fin peeking below the belly adds dimension. Draw far-side features smaller and with less contrast.
Color the character, not the category
A cartoon fish does not need realistic color. Choose a main hue that matches the mood, then a contrasting accent for the feature you exaggerated. A calm teal body with a coral tail makes the tail the event. A warm yellow body with a dark eye makes the face lead.
Repeat one shape from the silhouette inside the pattern. A round fish can use round spots; a sharp fish can use angled stripes. This repetition makes the character feel designed rather than decorated after the fact.