In case it wasnt shared, Krita is a free graphic program that has a very neat feature to create seamless patterns.
At begining you will need to go to settings> configure Krita and in the new opened window you select Display category to enable OpenGL.
When this done, you can press keyboard shortcut W to toggle between wrap around mode. It basically multiplies the image all over the canvas which you can easylie edit to create amazing textures.
This is sofar the easiest feature to create endless textures for tiled game graphic that i have found.