Offset is super easy. (Obviously this is coming from the post about seamless texturing)

You activate the Offset filter (Filter>Other>Offset)

Whatver size your image texture is, you just type into the fields half your actual size. In this case the image texture is 1024 x 1024, so my offset size is 512 & 512, then you hit "Okay". 

As you can see in the screenshot, the offset puts the outer edges together (in the middle). You would then go in and blend those edges together (with the Clone Stamp and/or the Heal tool) and when you have them just right, do the Offset again, re-check, Offset again, recheck. When you can see no evidence of any abrupt edges, you have a seamless texture. ;o)

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