Open up a new image with a white background, and use the draw tool () to draw a squiggle of some sort. Or if you like, use one of mine (right click and copy, then paste in PSP):
Now open up the image you want to add a pattern to, and go to ‘Layers’ > ‘New Raster Layer…’, and a box will pop up straight away. Change the ‘Opacity’ to about 15-20, and click ‘Ok’.
Go to where you change the colours you want to use (colour palette), and select the ‘Patterns’ feature (shown on the image below), by clicking the tiny black arrow and then the dotted square.
Now click the black arrow next to the ‘Current Pattern’, and find yours. Press ‘Ok’, then use the flood fill bucket () to add the pattern to your image.
Save your image! Here are my examples: