First what
you will need to do is create an image that you will want to use for a
seamless background (I recommend 100 x 100). I have created a series of
red balls as you can see....
Next
go to Filter / other / offset. In the offset window, select "wrap
around" and change the horizontal and verticle pixel values both to
50. This will cause your image to be re-arranged so that the edges of the
image will line up perfectly when tiled next to each other. However, (almost
always) seams will be formed in the middle of the image (quite obvious
in my example).
There
are a few ways in which you can deal with these seams. Many times you can
simply use the smudge or blur tool to remove the seams; or create something
new over the seams that would match the pattern. In my example, I simply
created new red circles of different sizes and placed them carefully so
that they covered the seams from view. This is all you have to do, and
the image will create a seamless background when tiled. Easier than you
thought it would be, wasn't it?