Open the picture you want to put the watermark on.

Use the Horizontal Type Tool to create what you want to have as your watermark.

Here are the two layers together.

Control-click on text layer, then Layer-Rasterize-Layer and then go to Edit-Free Transform. Once you do that, click on the upper-right corner and drag the layer up as shown here. Then grab the upper-left corner and drag it down. Double-click the layer to accept all the changes.

Lower the Opacity enough to allow the picture to be seen but not enough so that is not noticeable. I did it at 25 %.