Lay the hooked rug out onto a flat surface, such as a table top. Identify the areas which need repairing.
Cut a piece of matching yarn to measure 6 inches long.
Wrap the piece of yarn around a latch hook tool, placing the yarn underneath the latch.
Insert the hook and latch portion of the latch hook tool underneath one of the threads of the rug backing missing its yarn, and push the hook out through the neighboring hole. The wrapped yarn should be positioned on the other side of the rug thread, opposite from the hook and latch.
Wrap the ends of the yarn piece in between the hook and the latch, then pull the latch hook back down out of the thread. This will cause the yarn piece to become securely looped around the thread. Repeat this process with each section of the hooked rug that is missing pieces of yarn.
Fluff the new yarn pieces in the rug upward, and trim the yarn with scissors until they match the level of the other pieces of yarn in the rug.