Part 1 - Pattern
Choose a square portion of your scanned shape freeform composition. You can set the Marquee Selection tool for a square by using Fixed Ratio set to 1:1.
Be sure to choose an interesting, dynamic portion of your composition. You can crop quite tightly, if you wish.
When you have your chosen square area and cropped to it (Image/Crop), SAVE AS a new document <pattern.psd>.
Use Canvas Size with an upper-left lock. Make both the width and the height double the original dimensions, extending with white, so that your square of pattern is in the upper left corner of the newly enlarged canvas.
You can create a mirrored/rotated pattern element in multiple ways:
(1) Select just the upper left corner (with the square-set marquee). Copy and Paste. Use Edit/Transform to "Flip Horizontal" and then move the flipped block into the upper right quadrant. Copy and paste the upper right quadrant, and flip it vertically. Move it to the lower right quadrant. Copy and paste this, flip it horizontally, and move it to the lower left.
(2) Do the same set of copy/pastes as above, but use "rotate 90-degrees clockwise" or "counterclockwise" for each new quadrant.
(3) A different method - of your own invention - that produces a pattern block with good symmetry and a dynamic quality.
Use Image/Size to make your module 600x600 pixels in size with a print resolution of 300 pixels/inch.
Use Canvas Size, with an upper-left lock, to expand the canvas to 12 inches across by 8 inches high (white background fill). Copy, paste and arrange your pattern module so that you have six across and four down (24 total modules) without gaps. Flatten Image; save as <sec-name-pattern.psd>.
Duplicate the background layer; then use Image/Adjustments/Invert. This will produce a while lines on black ground version. Name the new layer <invert>.