Masking in flash

Letter Masking in Flash with Background

1. Open Adobe Flash CS3 
2. Select Flash File (Actionscript 2.0) 
3. Now you are in the page 
4. You are in layer 1 and rename the layer 1 as "background"
5. For this, double click on layer 1 and rename it "background"
6. For background, either you can choose rectangular tool or you can import picture .
7. For Rectangular tool 
8. You can use it from the tool box
9. For Picture 
10. Go to File
11. Select Import
12. Select Import to stage
13. Adjust the size of of picture using "Free Transform tool"
14. Select picture whichever you want for background
15. Choose OK or Open
16. Here I have used Rectangle box using "Rectangular Tool"
17. You are in layer 1 and frame 1
18. Press f6 on the frame 100
19. Insert new layer 2 rename it "Text1"
20. Remaining on layer 2 and frame 1, type the text
21. If u want colorful letter you can color it 
22. Press f6 on frame 50
23. Add another layer 3 and rename it "mask1"
24. Remaining on layer 3 and frame 1, draw a circle and put it in one end of the text
25. Press f6 on frame 50
26. You can drag the circle to another end or
27. You can enlarge the circle until it cover the text
28. Right click on layer 3 and select "Create Shape Tween"
29. Right click on the "mask1" and choose mask
30. Now press ctrl+enter to see how it work.

1. To add more fun effects
2. Add new layer 4, rename it "text2"
3. Press f6 on frame 51
4. Remaining on frame 51 type the text
5. Press f6 on frame 100
6. Add another layer 5 for masking 
7. Rename it as "mask2"
8. Press f6 on frame 51
9. Draw the circle and drag to one end of the circle
10. Press f6 on frame 100
11. You can drag the circle to another end or
12. You can enlarge it until it cover the text
13. Right click on the layer 5 and select "Create Shape Tween"
14. Right click on "mask2" and select mask
15. Now press ctrl+enter to see the effect.

NOTE : for another effect you have to add new layer for text and mask as in the second effect.

I have some pictures, I hope that will help you to do masking.