1 Start by creating a new RGB file, say, 1500 x 1500 px, with a transparent background. Using the rectangular-marquee tool, select a smaller square in the lower-left quadrant of the artwork, say 250 x 250 px. Fill the selection with your favorite neutral color, for example, mustard. Apply the Noise > Add Noise... filter (20% amount, uniform distribution, monochromatic). You may well use other parameters, but make the noise pattern as featureless as possible.
2 Apply one of the Pixelate filters, for example, Pointillize (cell size 20). The Crystallize filter also gives good results. Again, vary parameters to make the distribution of cells uniform.
3 Apply the Artistic > Colored Pencil... filter (width 11, pressure 5, brightness 25). The Cutout, Dry Brush, Sponge, and many other filters are also worth trying. Select a somewhat smaller area inside your square with, for example, the magnetic-lasso tool (feather 0, width 2 px, contrast 2%, frequency 100). Make your selection elaborate (straight lines may give unwanted stripes in the final pattern); when tracing, try to avoid making long boundaries along very dark or very light areas. It may be better to cut through cells instead. When done, try Select > Feather (radius 2 px; bigger radii or no feather at all may work better in some cases).
Alternatively, you can make no selection yourself, but let SymmetryShop apply the automatic selection (overlap 10-20%, feather radius 1.5 px (or more)).
4 Finally, choose File > Automate > Artlandia SymmetryShop... to put your seed image in repeat. In the Layout list, choose the 4 x 4 grid with a vertical shift. Smaller grids (3 x 3) also work satisfactorily, as do simple half-drop repeats. If using a built-in layout, shrink the control path by 20 or 30% so that repeated units overlap without gaps.