There's a tutorial (not mine) linked in thread #13 in this post here:
Using GIMP
That's for a basic text-only one.
In post #15 I describe how I use brushes to add images to mine.
If you want a text-only watermark, I believe many people use Picasa to create theirs - it's less of a learning curve.
Two more threads with chat and helpful links:
WATERMARK
How to add a watermark to your photos?