Step 20 (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 final sketch lines that you may have accidentally erased.
Final Step (optional): Add some shading throughout your mosasaurus drawing for extra detail. Use a medium value all over the body. Wherever there are detail lines, like around the eyes, use a darker value. Use a slightly lighter value above the brow and on top of the body to create highlights. The mosasaurus from Jurassic World was gray, but no one really knows what they looked like, so you can shade yours any way you want! Don't worry about shading too smoothly. The rough value gives the skin a gritty texture.
Shading can be time-consuming, so be patient and take breaks. Make the flippers on the other side of the body darker. Add a few more triangle-like shapes on top of the dinosaur's body for more spikes. Use a darker value on the underside of the body to give it more dimension and volume. Remember to pause the video after each step to draw at your own pace.