Measure the inside of the cuffs widthwise and lengthwise. Cut a piece of fabric with a seam allowance all around.
Iron all four edges under so that it equals the exact dimensions you just measured.
Sew the fabric to the inside of the cuff. It helps to turn the cuff inside out. (If you are covering any buttonholes, you may have to make new ones in your fabric.)
Choose a fabric for the inside of the collar. I covered the placket (I think that's what that strip of fabric is called) - not the entire collar. Measure the placket and once again cut fabric to fit after allowing for all four edges to be ironed under.
Pin then sew in place.
This is my old-new white shirt.