Seamless tiles

The core skill of tile art is making tiles that connect. A ground tile has to line up with a copy of itself on every side, so when it repeats across a floor you see a continuous surface, not a grid of obvious squares. That means the pixels along the left edge must flow into the right edge, and top into bottom.

Wrapping

A tile that connects to itself on all four sides is "seamless" or "wrapping". Test it by placing several copies side by side: if you can spot where one tile ends and the next begins, the edges do not match yet. Avoid a strong feature dead-center that draws the eye to the repetition.