In earlier posts I’ve talked about doing seamless repeats in Photoshop. Making an Illustrator design into a seamless repeat is also quite straightforward. And, since Illustrator designs are vectors, they can be infinitely scaled without losing any detail.
I’ll go through the steps using this really simple design. In the picture below, the artboard bounds are defined by the white square, and this will also be the bounds for the repeat design.
Color On Cloth 