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Use a small drinking glass or a compass to make the circle. If you are using a standard paper towel tube, a 4-inch (10.16-centimeter) circle ill be just the right size. You can make the cone any color you want, but red or orange are the most popular colors. If you are having troubles figuring out here to cut the slit, use a ruler and a pencil to draw an X across the circle. Cut along one of the X's arms towards the middle. Overlap the two edges of the slit until the circle turns into a cone. It needs to be big enough to sit on top of the tube without falling through. Tape or glue the cone together. A glue stick or double-sided tape will work the best here. Don't use hot glue; it is too bulky for this step. Draw a ring of hot glue along the rim of the paper towel tube. Place the cone on top and press it into the glue. Work quickly; hot glue sets up fast. You can use regular tacky glue, but you'll have to wait for it to dry. Make sure that you are gluing the cone to the end that does not have the slits.
Trace a circle onto a sheet of cardstock or construction paper. Cut the circle out, then cut a slit into the middle. Roll the circle into a cone shape, then secure it. Hot glue the cone to the top of the toilet paper roll.