I do it using the "process multiple photos" option in PSE. Open all the photos you want to be watermarked, edit them, then go to File>Process Multiple Files. On the left, you'll see all your options for saving. Set those where you want the photo to be saved, and how you want them to be saved. Then on the right, you'll see the watermark option at the bottom of the list of additions. Just choose a font, font size (10-12 is good for saving for web), and the color and opacity. Hit the button and PSE does the rest of the work! I assume it works the same in PS.