Cut out a 3-by-4-inch rectangle on one long side of the shoebox. The cutout should be centered on the side and leave the edges of the box intact. Trace the base of the flashlight on the center of the opposite side and cut out the traced section.
Put the flashlight through the hole so the handle and on-off switch extend outside the box. The light should be inside the box pointed toward the rectangle cutout on the opposite side.
Tape the lid of the shoebox onto the box, leaving the flashlight positioned as described in Step 2. Seal all the way around the edges to prevent light from escaping.
Select the constellation your planetarium will feature. Cut a 3 1/2-by-4 1/2-inch rectangle of black construction paper. Draw the constellation outline on the construction paper using white chalk. Mark the main stars with the chalk. Poke the needle through the black paper at the center of each star.
Tape the construction paper constellation over the cut-out rectangle on the side of the box. The chalk-marked side of the diagram should face the inside of the box.
Go to a darkened room. Turn on the flashlight and point the planetarium toward a blank wall or the ceiling to display the constellation.