The principle of every glassy effect lies in lighting of an object. A sourse of light gives a highlighted area and shadowed area to almost everything in this world, especially when object's material is glass or plastic. I know sounds stupidly, but it's all the theory you should know. All the following are the methods to bring this theory to your digital 2D creations. The basic principle of creating glassy effect in Photoshop lies in bright and dark gradients begin one out of another like on the following object scheme. This tutorial demonstrates how to create the glassy effect in the simplest terms.