First Step, create the Watermark. Don't worry about opacity of the watermark, just create the image that will be the watermark. You will need to make sure that the background is alpha(Transparent)

Second Step, Open the image you want to watermark.

Third Step Import the watermark created in step one as a new layer. Make sure this layer is on top, then adjust the opacity in the layer dialog to the desired opactiy for your watermark.

Fourth Step, save the image as the desired final format. Flatten the image while saving it.

Really if I go much more detailed than this you are asking me for a primer on image creation and editing, which is an entire book in itself, and something you should know if you were already doing this in photoshop.

How about asking specific questions on what is giving you problems?

Seablade