Begin this practice by open photoshop (photoshop7.0 for this tutorial)  and go to File >> New set job space as 300 x 150 pixels set resolution at 72 pixels/inch in RGB Mode. In new layer (layer1) use rounded rectangular tool to create round bottom curve in layer 1. At layer 1 go to pallete path and make selection. Fill black color in this layer and go to menu select deselect. After that choose effect style as inner glow set blend mode as screen and change opacity to 100%.
 
 
Please remind me at rjairak@gmail.com if you can't view this flash vdo. Thank you.