The simplest way (if you dont need special details) is to use the automatic effect in most good graphics program "make seamless".
Gimp has that for example.

If you are not an artists, its good enough for most uses.

Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html