First, you will need an image you would like to use to create your watermark. If you would like to use your signature, try writing it large on a blank piece of white paper with a fine-tip black Sharpie. Your image will need to be black and white to work right. In this case, I'm going to use my logo. Once you've got it, open up Photoshop, then go to File>Open and locate your image.
Once you have your image up, make sure it is nice and clean. Remember, anything that is black will show up in your watermark. The white background will be transparent, so there is no need to try and remove it.
You will want a fairly large image for your watermark. It is easy to make smaller, but will get distorted if you try to make it larger. However, you don't want it too large or it can be difficult to work with. On the menu bar, go to Image>Image Size.
Once the Image Size screen opens, you can adjust the width or height. Here, I changed the width to 1500 pixels. That is usually a fairly good size for the images I make. Also, note the resolution. I kept it on the high end of 300 Pixels/Inch. That should keep it nice and clean. Be careful not to go with too low a resolution. I would suggest 170 or higher.
Now, you're ready to create your watermark. On the menu bar, click on Edit and go down to Define Brush Preset.
Give your brush a name. It should be easy for you to find it when you need it, but I suggest giving your watermark a unique name.
Now, you're ready to use your newly created watermark! Open an image you would like to use. Select the Brush Tool from your toolbar.
When the Brush Presets box opens, scroll down to the bottom. Your watermark should appear at the very bottom.
You will probably want to make some adjustments once you place your watermark, so click on the "Create a new layer" icon at the bottom of your layers panel. This will place your watermark on its own layer and not effect the main image.
Now, all you have to do is click your mouse once to place your watermark.
If you find your watermark too distracting, adjust the Opacity at the top right corner of the Layers panel. In this case, I went with 50%.
If you like, you can set the color of your watermark. Click on the Set foreground color box, and choose a color from the dialog box that comes up.
You can also adjust the size of your watermark. Select the brush tool, then click on the drop down arrow in the Brush Preset box. Here you can either type in a size or use the Size slider.
Once you get the size and color you would like to use, locate where you would like to place your watermark and click your mouse once. Because it is on its own layer, you can move it around with the Move tool at the top of your tool bar until you are satisfied with its placement. To resize your watermark, hold down the Control and T keys at the same time. Your watermark will appear with handles you can use. When you have it sized, just hit the Enter key.
Before you save your image with the watermark, you will need to bring the two layers into one. On the menu bar, click on Layers and scroll down to Flatten Image. I suggest using File>Save As to save your image. This way, you can rename your image and keep the original in tact.