You have to start first by making your drawings in cubism technique. That is representing every shapes with either rectangle, square, circles, triangles, etc.
Cubism is the art of using the elementary shapes aforesaid to represent images and shapes in drawing and design concept.
After the application of this technique, you can then go ahead to develop your drawing by defining the forms and shade.
When drawing the leaves, close one eye and get the feel of the shading pattern appropriate for the leave folliage.