Drawing glass, and other transparent things, is not as hard as it may seem. What makes it seem difficult is not because of the subject itself; it’s because of how we think about it. Our biggest hurdle to overcome when drawing is to overcome what our brains are telling us about the object. Rather than thinking so much, we should be observing and analysing instead. Use your eyes, and not your memory.
Bottles are fun to draw because you stack simple shapes to form them. The label on any curved container will echo the curvature of that surface. Make labels similar in curvature to the bottom or top edge closest to it. You can draw ellipses inside the container to help guide you.
Year 8 have used this technique to create the wonderful images below.