This article is reposted from http://www.photoshoptextures.com.
The direct link is http://www.photoshoptextures.com/texture-tutorials/seamless-textures.htm
How to Create a Seamless Texture in Photoshop
To begin, you need to have a perfectly square texture image that you want to make seamless. You can download the one I use for this tutorial here > Bronze Texture.
If you don't have a perfectly square image, I'll explain briefly how to make an image square.
How to make a texture perfectly square:
First open you document. Now select the Crop tool or press the "C" key and Photoshop will automatically select it for you. With the Crop tool, place the cursor in one of the corners of your image. Now, holding down the Shift Key, and drag the Crop square to the opposite corner. You notice that the selection is forced to be a square. After you let go of both the mouse and the select key, you double click in the center of the selection. Tadaaa, your texture image is now cropped to a perfect square.
For the purposes of this tutorial, you should resize the image so you can easily offset the pixels when needed. To do this, from the Menu bar, go to Image>Image Size or (Alt+Ctrl+I)....In the Image Size Window, change the Pixel Dimensions to something easy to remember, like 700 pixels (Height) x 700 pixels (Width). Now we're ready to start the Tutorial :)
Open the file. Download the Bronze Texture and Open it in Photoshop. The actual name of this file is "texture-before7.jpg"
Offset the image. From the menu bar, go to Filter> Other> Offset...
In the Offset Window, adjust the Pixels to half of what the pixel dimensions are of your image. The image we're using is 700x700 pixels, so we offset it 350 pixels to the right, the 350 pixels down. Leave the "Wrap Around" selection selected, and the Preview Box selected to make sure you're doing it write.
Hit OK, and your image should look like the image below.
Clean the image of the Offset Lines. To do this we are going to use the handy Clone Stamp Tool . You can select it from the tool box on the left or Press the "S" key and Photoshop will select it for you.
After you've selected the Clone Stamp tool, make sure your brush size it appropriate for the image, you want a faily large bruse that has soft edges. Here, I chose a soft brush with a diameter of 45 pixels. This option is typically directly below the menu bar. If it's not, go to Window> and select Options. I usually adjust my Opacity and Flow of the Brush to about 85% each.
Now it's time to start using the Clone Stamp Tool. How this tool works is that you hold down the Alt key and select (Click on) a specific spot in your texture. Then you let go of the Alt key and move the cursor to a spot where you want to copy that last spot you clicked on. Now, you click and hold down and paint the area. What happens is you're virtually painting a copy of the area you previously selected.
So, for our image, we want to get rid of the lines, and we don't want that dark area in the middle of the texture to be so dark. The image below shows you and Example of a place I used the Clone Stamp Tool.
Keep using the Clone Stamp tool and select different areas of the image. Mix it up a little. The goal here is to not make the image look repetitive...we want seamless. And to get rid of the lines that we got from offsetting.
The image below shows what my seamless texture looked like after I used the Clone Stamp Tool
Save the Image...File>Save As. And You're DONE! You've made a perfectly seamless texture.
The images below show the difference when you tile them and apply it to a 3D plane. And what a difference it is! Download the Seamless Texture After.