I have recently finished a sequence of three classes with my students about the process of drawing a small plant from observation / photograph.
The idea was to help the students learning to work in cycles of information.
Drawing foliage isn't the easiest thing on earth. It demands a lot of attention and I believe it is specially difficult to draw leaves that come forward, into the viewer's direction. Other difficult aspect is to get the right size of the whole plant.
So, we began by doing a broad, loose sketch. Then we tried to correct some angles and sizes of individual leaves, adding some details and making the values darker little by little (at the same time we turn the highlights brighter).
I scanned the drawing whenever I felt each cycle was finished.
Here's a tip: when drawing a plant, make sure to take a picture at the end of the first session, because the leaves will be different the next day - so you can draw from the picture from this point on.