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I remember searching for tutorials for making seamless patterns in Photoshop¹ all the time back in the day.
It’s fun to see this little website for building repeating patterns as its one job. It does everything you’d expect: pick a background, drag some decorations onto it and position them (overlapping an edge is fine, which is the hard part, and makes the designs look less griddy), color them, and away you go.
The trick is to make a square with a design in it, then Filter > Other > Offset of half the width/height to make the seamless repeating edges work, then add a bit more in gaps.