Tileable textures create in Photoshop

Here I have a image texture (400 x 300 pixels) in the Layers Panel which is currently not ready to be used as a tileable texture.

Now, go to Filter > Others > Offset… It will bring up the Offset dialog box.

Go ahead and Offset the image horizontally by 400 pixels and vertically by 300 pixels. Please make sure that under Undefined Areas group, Warp around option is selected.

For suitable offset value, check the dimensions of your image and use exactly the half of your image dimensions as Offset value. For example, I am using an image having dimensions 400 x 300 pixels so I used 400 x 300 as my offset value.

Note: If you used different Texture, then...
After offsetting your image, you can easily see the two seams visible on our image. Now, select Clone Stamp Tool (S) from toolbar and remove both the seams from your image.

Now you can use your image as a tileable texture for your games and other projects.