Step 15 (optional): 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 of the final sketch lines that you may have accidentally erased.
Final Step (optional): Add some shading to your narwhal 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. You can add even more value throughout your narwhal drawing for extra detail. Narwhals have dark heads, so push harder with your pencil in this area to get a darker value. You can add the value lightly at first, then gradually build up to the level of darkness that you want.
Narwhals have a splotchy pattern on the tops of their bodies, so leave some blank sections as you add the value here. The dark area becomes more scattered near the bottom and turns into spots. Add these spots along the middle of the body. Don't worry about adding the spots perfectly. Just draw random blotches all over the body. Just make sure that the blotches are more frequent and are closer together at the top than at the bottom.
Adding the splotches all over the body can be very time-consuming, so be patient and take breaks. The flipper and the tail should be very dark with hardly any white on them. Vary the size, shape and value of the spots to make the pattern more random and less systematic. It's always a good idea to use reference if you want a more accurate drawing. Don't forget to pause the "How to Draw a Narwhal" video after each step to draw at your own pace.