Any size straw will do so long as the small drill bit matches.
Cut the carrot to 4 or 5 inches long.
Compare the length of the 1/2 inch drill bit to to length of the carrot. You want to know how deep to go to leave about an inch at the end of the carrot.
Slowly by hand, drill into the carrot. Make sure to go slow and straight and not too deep.
A nice straight hole. Be sure to leave an inch at the "bottom" of the carrot.
Use the small bit to drill a hole perpendicular to the first hole you made.
Slide the straw into the small hole then use the scissors to trim it to fit the carrot.
You are almost done. Slip the string through the straw and tie into a loop.
Hang in a window and fill with water. Do not use plant food, just keep adding water twice a day. An eye dropper may make it easier. Start with a carrot with a good amount of green on it.
The new sprouts will start to grow up the sides of the carrot. They are genetically programmed to grow away from gravity. Cool science and a nice houseplant to boot.