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: Add some shading to your drawing for more detail. Nurse sharks don't really have a distinctive pattern on their skin, so use a medium value for the entire body. Use strokes that go in the same direction throughout the body to avoid creating a criss-cross pattern. Make the underside of the body lighter than the top part. Vary the pressure on your pencil to get different degrees of tonal value. Use a slightly darker value when you shade the shark's tail. For a more detailed guide on how to shade, check out this tutorial: How to shade.
Add the value to your nurse shark drawing lightly at first, then gradually build up to the level of darkness that you like. Add slightly darker values around the edges to create shadows and give the figure more dimension and volume. Shading can be time-consuming, so be patient and take breaks. Continue shading gradually, adding a darker value slowly until you're happy with the results. It's always a good idea to use reference as you draw. Don't forget to pause the video to draw at your own pace.