Start by making a new layer, then make a round selection. To make it a circle, press the Shift-key while dragging. Now fill the selection with black. Airbrush a big highlight in the direction the light is coming from (usually up) and a small highlight on the opposite side. Apply "Layer Effects" and put some "Inner Glow" in the glass ball layer, use white for the color, and set "Blur" to 1 or 2, Opacity to 100%, Intensity to about 40-50 and click "Done." Now load the glass ball layer transparency as new selection and apply Spherize with 100 as the setting. That's it. Now you have a glass ball.

The second step in making a glass ball is making it transparent. It's really easy, just set the glass ball layer blend mode to "Screen". See, I told you it'd be simple. Of course the "Screen" trick doesn't work on very light backgrounds, so either don't use light backgrounds, or you could use a little work-around to this problem. The work-around is making a layer that's the same size as your glass ball, filling it with black, and placing it behind the glass ball. Then set the black layer opacity to somewhere around 10-20%. Now you can see through the glass ball, and you can see the glass ball even on a white background.

The third step is coloring the glass ball. This step is voluntary, but you like having colored glass balls, don't ya ^_^. As the last step, this one's going to be really easy as well. Just go to "Image>Adjust" -menu, click on Hue/Saturation, then check the "Colorize" -checkbox and adjust the hue and saturation to your liking.