Take a pen or pencil, and a sheet of paper. Using one of her existing rings, draw the outer ring, then draw the inner ring. Make sure you’re using a ring that she currently wears on “ring” finger (the one next to the pinky on the left hand). Take this to a jeweler for his or her expertise. Keep in mind that a ring should be comfortable — a little snug but with some room. If it falls between two sizes, size up.