This tutorial will give you an easy way to tile textures in photoshop. You can duplicate it in any program that supports a tool like the lasso tool or the healing brush, since you can reproduce the offset filter manually.
I am working with a fairly small texture here but normally you would be working with a much larger size. In that case don't forget to use a bigger brush size and to use the lasso tool to make larger 'patches' as well.
Also make sure you keep in mind that your job is not only to make the edges seamless but to try and remove any possible repeating patterns that would break the effect.
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