How do you replace the string on a MTD PRO 4 cycle weed eater?

In general- secure a replacement starter cord. Remove the cord starter reel, Wind the spring driven reel about 5 turns, aligning the cord hole in the reel with the channel for the cord. While holding the reel against the spring, insert the starter cord, tie a knot on the cord inside the reel, let the spring gradually turn the reel until the cord Is wrapped on the reel, replace the starter wheel unit on the weed eater, replace the screws that hold it in place.