If you want the edges to tile seamlessly, simply free transform the shapes you're working with beyond the Photoshop canvas by about 25 to 100 pixels on each side. Once you're done with normal detailing, you simply load the flattened normal map (e.g. save as Targa) into DDO with/without a mesh to preview it on. Additional…
Tiling is a matter of understanding how tiles function. There's tons of video content out there explaining how to make a tileable texture, but probably not as many for NDO. You need to define how many repetitions you want within the tile: essentially, how many corrugated pieces of metal need to be within your texture. To…