aseanz wrote one in one of his threads a while back but ill give you about what he said...
1. trace the oulines of all the differen shades of the colors
2. fill in the dark colors, then lighter, then lighter, untill you get to white...
3. take a tort or stump and blend the line...basically trace your lines...
4 add highlights to look like glass
i think that was about it but im sure alfred can help you more