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I am sorry to tell you that you can't do it in SketchBook. The only program that has the tools to create a pattern is Photoshop.
@david.lau That is incorrect. There are many other options.
@Anonymous My recommendations is to try Krita. It's another digital painting application that is as good if not better than Photoshop for digital hand-painted textures. Also it's free.
https://krita.org/en/
Procreate on the iPad is great for seamless repeat pattern making there is tutorials on skill share, and YouTube, also within spoonflower.com on how to do this.
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