1First make a new image.
Any size will do.
Remember though we will be cutting parts out of it so make it large enough to accommodate your dimensions.
On the first layer start the clouds plugin "Solid Noise".
Located under filters>render>clouds.
Set detail to 15.
X to 16.
And Y to it's lowest.
Be sure to check off Turbulent, and Tilable.
Note our final image will not be tilable.