Sai

Note that your text should not have a lot of anti-aliasing, so using a paint brush to start isn't going to work well with this method. Just use the standard G-Pen if you're doing this by hand, or, just use the text tool and whichever font you prefer.

Once that's done, take your magic wand tool, and select all the black. Here are the magic wand settings I'm using to make the selections:

All selected?

Good.

Now, find a brush with a scattering/tone scraping effect. I use one like this.

You can theoretically use any colors you want for this next part, but I'd recommend pastels as they tend to blend better.

Either way, let's add some color to the text.

Once that's finished,

You're going to want to go to Layer Property, and Border Effect

You'll be given an option of choosing color and thickness. Choose black, and go for at least a 5 in thickness. Adjust per your own preferences.

Now create a layer beneath your sig layer, and merge the sig down onto the blank layer.

This effectively 'locks in' the border effect, which is exactly what we want.

Hooray, you've finished your watermark!

Now let's place that bad boy into your finished piece.

You'll get the best mileage out of a mark if you can place it over a spot that isn't black of white, since you'll get better blending options that way. My preference is for Overlay.

From here, I'll adjust the opacity to around 20-25, depending on the image.

If you don't have a spot to use overlay, however, there's a couple other options. For white, there's Linear Burn, which imho doesn't look as good, but it still works in a pinch.

And for lots of black, you have Linear Light

Either way, you're in business!

EDIT since this has escaped my usual circles, and folks aren't as familiar with my personal usage:

An example of one of my own finished pieces, with watermark, so you can see what I mean about 'relatively unobtrusive'-- I try to at least use them as framing devices, or let them work with the image somehow (or, at the very least, not actively against it).

I know it's a bummer for some people to "ruin" their work with watermarks, which is part of the reason I developed this mark in particular. Its disruption is about as minimal as I can make it while still letting it serve its intended purpose.