STEP 2: Displacement Map
Crop your orthographic photo, color correct it, and resize it to 1024 x 1024. This will be your color map. Using the color map as reference, block out the main areas in different shades of grey (top left). Using Photoshop's paintbrush tool, you can layer in the furrows in the metal sheeting. A few random marks with a soft brush help to create some irregularity.
Wait until the last stage to paint in details like the bolts and the rusted nail. You can also add texture by cutting and pasting parts of the original image, but be careful to tone down the value contrast.