Pick the text you'd like to use. Type out your mark in white. For this we'll use the word Sample. I use size 8 or 6, something small, but still readable.

Selections – From Vector Object

Selections – Modify-Expand – Expand by 2

New Raster Layer, put the vector text layer over the top

Flood Fill the Raster layer with black – Deselect

Merge the two layers together as a Raster

Save as a PSP image.

C/P to your image and place where you want. Change the Layer style to Overlay

Bam! You've just created your own watermark!  If you have questions, please email me, keep the rippers away!