French cuffs are different from regular cuffs. Instead of the buttonhole on the cuff folding over the button, the French cuff shirt has two buttonholes and a cufflink. The cufflink acts like a button on a regular shirt by holding the two ends of the sleeve together. Regular dress shirts can easily be turned into French cuffed shirts if you have a sewing machine that you can make new buttonholes with.
Cut the button off of the sleeve of the regular shirt.
Hold the ends of the cuffs together so they overlap each other. The second buttonhole should line up with the first button hole. Mark this place with a fabric pen.
Put the buttonhole foot on your sewing machine and select a button of the proper size to make a hole the same size as the first button hole.
Set the sewing machine stitch width to zero.
Place the cuff that needs a new buttonhole under the buttonhole foot. Sew the new buttonhole on your shirt following your sewing machine's buttonhole instructions.
Place a cufflink between both buttonholes to hold the ends of the sleeves together.