Hi Sophie,
If you step to sketching complex surfaces and blends, I recommend first mastering basic use of materials.
If you know how to draw reflective surfaces, you can already start applying that to complex surfaces as well which makes them much more legible besides the contour and section cut lines you usually draw.
A good surface comes from a good division and proportioning of volumes. It is a good idea for you to start blending volumes. First determine for both volumes where the blend is going to flow into the volume, draw a line there, and then draw the section lines flowing from the first volume to the other. Then also determine for other volumes, if there are any, the in-between surface. From there you can start shading.
Keep posting and we will stay in the loop