Make a new document. Make the height a lot longer than you want it to be. I chose 150x350 pixels. Follow the matrix tutorial to get the background.
When you're finished with the matrix tutorial, hop to ImageReady. We'll be doing all our animating in there. When you're there, go to Windows > Animation. Press the Duplicate Frame button. Now there should be two frames. Select the second frame, and select the Move Tool. Hold down Shift, and drag the matrix layer down the workspace.
Now, go to Select > Load Selection, and load the transparency of the matrix layer (Ctrl+Click+Matrix Layer). Then go to Image > Crop. Then on the animation palette, click the Tween button.
Finally, go to File > Save Optimized. Make sure you're saving as a *.gif. Name it whatever you want, and you're done! Congratulations.
