For a pattern to be truly seamless, the edges of the design must match exactly on all sides. That means elements on one edge line up with the opposite edge when the design is repeated. When this alignment is correct, no breaks or edges show up in the repeating result.
In digital design tools, artists often:
Place key elements away from the edge at first
Shift pieces so they wrap from one side to the opposite
Fill in gaps so the tile flows smoothly
This ensures the repetition looks continuous.