There is a tool, but I have always used a fine tipped tweezers. I would imagine a small crocheting needle would work as well as the knot tying tool. There are a bunch of youtube video's on the subject.
1)Make a loop in the fibers.
2)Put tip of tweezers through loop.
3)Pinch fibers with tweezers tip
4)Pull fibers back through loop with tweezers tip.