Using the electric hand drill and a 3/16” bit, widen the holes in the plastic tubing so that the dowel fits snuggly through the holes. You may need an adult to assist you. Then, secure the plastic tubing cap onto the end of the tube that is closest to the holes you just made.
Be very careful when using an electric hand drill! Wear safety glasses, place your hands as far away as possible from the drill point, and make sure that what you are drilling into is stable.