Step 20: For a cleaner look, erase as much as you can of the initial guide lines. Don't worry about erasing all of the guides. It's okay to leave some behind. Re-draw any final sketch lines you may have accidentally erased.
Final Step (optional): Add some shading to your drawing to give it more dimension and volume. Pick the direction of the light source when shading so that the shadows are consistent with it. Vary the pressure on your pencil to get different degrees of tonal value.
Add a cast shadow underneath. This helps ground the gull so it doesn't appear to be floating.
You can add even more value throughout your drawing for extra detail. Gulls have a variety of patterns on their feathers. For a typical one, simply add a dark value to the wings and tail. Within the wing, leave the edges of the feathers blank along with a striped pattern on the tips for more variety. Use a light value for the legs and beak too. It's always a good idea to use reference as you draw.