Start with a double-threaded needle. Bring the needle up through the wrong side of the fabric and one hole of the button.
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Bring the needle back down through an opposite hole. Repeat six times with the same two holes. Move on to the next pair, forming an equal sign. (If the holes on the other buttons of the garment are paired in an X pattern, match that instead.)
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Tie a secure knot by passing the needle straight through the stitches on the wrong side of the fabric. Pull the thread through but stop short to form a loop. Insert the needle through the loop and pull tight. Repeat.