Photoshop - Creating Seamless Textures
This tutorial will walk you through the common way how to create a seamless texture in Photoshop.
I created the tutorial using Photoshop CC, but it should work very similarly in all versions.
This only works on textures, not patterns.
Textures are things like granite, dirt, or bark.
A pattern is something like lace, or anything with a design on it.