Masking, in itself, is a very simple process. You take one image, (the mask), and you stick something under it, (the masked). Whatever the shape of the mask is, is how the masked will look. Kinda like cutting a shape out of a piece of paper and laying the paper cutout on top of another sheet of a different color. Only the shape of the mask shows the shape of the underlying layer.

1. The first thing you want to do is create your mask, or the shape that will contain the mask. After you do this, Convert it to a movie clip.

2. Click Frame 20, and make a frame in the layer you put your mask.

3. Add a new layer udner the first one, and make the shape you want to put in the mask. Convert it to a symbol as well.

4. Drag your shape to the very left of your mask.Put a keyframe on frame 20, and drag the shape to the very right. Then create a motion tween.

5. Finally, Right click the 2nd layer and Mask it. This will finish the masking process. Press Ctrl+Enter to view your animation!