Step 30: Use the curved lines under the body as guides to draw the wyvern's tail. First darken the lines and don't overlap the shape of the wings. At the tip of the tail, draw an arrow-like shape using a couple of angled lines. Draw a series of short, angled lines along the tail for more spikes. The spikes curve behind the tail at the top. You can make them bigger if you'd like or not draw them at all.
Step 31: 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 wyvern drawing for more detail. Use a dark value for the shadows to give the figure more dimension and volume. Pick the direction of the light source when shading so that the shadows are consistent with it. Now use a medium value to shade the rest of the body. Vary the pressure on your pencil to get different degrees of tonal value. For a more detailed guide on how to shade, check out this tutorial: How to shade.
Don't worry about shading too smoothly; the rough value gives the skin texture. Wyverns are mythical, so you can shade yours differently if you'd like. You can even color it! But shading can be time-consuming, so be patient and take breaks. If you make the value on the body as dark as the shadows, make the shadows darker.
Add some lines to the wings for veins. Make the spikes darker and draw a few lines on the chest for scales. If you'd like, you can add stripes to your wyvern using a darker value. For a more rough, almost granite-like texture, add squiggly lines all over the body. Don't overthink it. Just add random squiggly lines all over the body. Don't forget to pause the video to draw at your own pace.