There are two ways of visualizing what you want to sketch, seeing shape and structure. Block in your sketch using shapes then refine your drawing by double checking that the structure is accurately observed.
A curling ribbon is a good model to begin thinking about surface shapes. Train your eye to catch the shapes formed by the green and blue sides. Each section has distinct angles and curves that are more easily seen and drawn in isolation from the rest of the ribbon.
Once you have blocked in the drawing with shapes, back up and observe the structure. The near and the far edges are continuous lines. The purple and blue edges do not have to be the same shape. You also see the surface of the ribbon between the edges at every turn (green lines).
If you are drawing a leaf, start by visualizing the top and bottom surfaces. See the top and the bottom surfaces as individual shapes. Join them to create a leaf. Then draw the close edge with a heavier line to connect the parts of the leaf and to emphasize distance and space.
Now think about the structure. To make sure your mid-vein and the far side of the leaf emerge at the right spot, imagine you can see through the leaf and follow the curves of the lines that are blocked from your sight. These should make smooth curves (or in some cases loops).
