First of all you need to choose your original pattern quite carefully. The original image should not have light and dark sections, the tone and pattern should be roughly the same throughout. I have chosen to use Rain Texture by FunnyBunny-stock
Open up the image that you want to use – make sure the image is on it’s own layer (not the background layer) and make the canvas twice as big.
Use the Marquee tool to section off the bottom half of the image.
With the marquee selection still active. Copy, cut and paste, either using the edit menu or using the keyboard short cuts. (Keyboard shortcuts are; Copy – Cmd C, Cut – Cmd X, Paste – Cmd V (Use Ctrl instead of command if you are on a PC).)
You will now have half the image on one layer and half on another.
Your top layer, which was originally the bottom of the image should be placed at the top of the canvas and the top of the image placed at the bottom. Line them up so the 2 edges meet.
Take the top image and keeping the edges of the image inline, allow it to overlap the bottom image slightly to a point where the pattern is close if possible. You can see the overlap below where the blue arrow is.
Zoom into the pattern and the point where the images overlap.
Slowly work your way along the line using a combination of the eraser and the clone tool to adjust the overlap until you can’t see a visible line. Work right the way to the left and the right of the line but don’t change the edges of the tile as these will line up perfectly if you leave them as they are. The quality of the end result will largely depend on how well you can blend the overlap here, so take your time to complete this step.
Once you are happy with the overlap and you can no longer see a visible line, select both layers in the layers palette and merge them together. A quick way to do this is to press shift to select more than one layer in the layers palette and use the keyboard shortcut Cmd E or Ctrl E to merge the layers.
Select the marquee tool and draw over the left of the pattern. Copy Cut and paste the left of the image and place this on the right and the original right side onto the left.
Like before line up the patterns until you are happy with the overlap. (What you see here will also be the final size of your tile).
Zoom into the vertical overlap line and repeat the earlier steps to create a seamless pattern, take care not to change the area at the top and bottom of the image at the edges of the tile.
Once you are happy with the look of the tile, merge the left and the right side of the pattern and you have the final tile.
To check that it works as a seamless tile, duplicate the layer and line up the tile. You should have your final seamless pattern.
Steps to Save the tile in Photoshop to use as a background pattern:
To use this in Photoshop as a background pattern copy and paste the single tile into a new document. To do this draw with the marquee around it and copy and then select a new document and paste. This will create a document at the exact size of your tile.
Choose edit and define pattern.
Give your pattern a name and click ok.
Your new pattern will now be available for you to use from the pain bucket pattern menu and from the pattern overlay menu.