Summarize this article:

You can use a ballpoint pen, a toothpick, or a wooden skewer to do this. If you are a child, ask an adult to help you with this step. To find the center, flip the plate over so the back is facing you. Draw a huge X across the back of the clock. Make sure that the points of the X are touching the edges of the clock. The center of the X is the center of the clock. Line up the bottom of the arrows up first, then punch the hole through both arrows at the same time. This will make it more even. You can use a hole puncher to make this easier. If you don't have a hole punch, you can poke the hole using a ballpoint pen. Make sure that the hole in each arrow lines up with the hole in the clock. The pointy end should be coming out of the back of the clock. Separate the prongs first, then flatten them against the plate. You can use a butter knife to pull the prongs apart. If you are a child, ask an adult to help you. Brads can be very sharp. Turn the clock back over so that you can see the numbers again. You can now move the arrows around. Try not to pull or tug too hard, or the paper will rip.

Summary:
Poke a hole in the center of the clock. Punch a hole in the bottom of each arrow. Place the arrows on the clock. Stick the brad through the holes. Turn the cock over and flatten the prongs. Use your clock.