Build a base for the trebuchet. This supports the movement of the device and needs to be solid. This is a small trebuchet, so a thin piece of wood about 5-by-7 inches is sufficient. Glue or tape two Popsicle sticks perpendicular to each other on the wood about three inches apart.
Create an A-frame on each side of the base. Glue or tape two Popsicle sticks to form a triangle from one side of the frame. Repeat on the other side. Tape or glue another Popsicle stick to each triangle to anchor the frame. Attach these sticks near the base on both sides.
Attach a second Popsicle strip directly above to further anchor the A-frame.
Straddle the two frames with the dowel, laying it at the front of the trebuchet. Anchor the dowel with another Popsicle stick across the side of the A-frame. This stick will be at an angle. The front is over the dowel. The back meets the previously laid Popsicle stick anchor.
Tape two Popsicle sticks together flat side to flat side, so you have a double Popsicle stick.
Slide the double Popsicle stick into the V at the top of the frame, flat side to the back. Tape or glue firmly in place.
Create the arm for the trebuchet. Glue three Popsicle sticks together flat side to flat side so you have a triple thick Popsicle stick.
Cut a Popsicle stick in half. Set it end on end with a full Popsicle stick and glue or tape together to make a longer stick. Glue or tape the longer stick to the triple thick stick, with ends jutting out equidistant from each end.
Glue or tape a Popsicle stick to the opposite side of the triple thick stick to match the length of the longer stick.
Glue a bottle cap behind this stick. The cap should be attached firmly to the triple thick stick.
Slide the arm under the double Popsicle sticks at the top of the trebuchet and over the dowel at the bottom of the front.
Attach rubber bands from the top of the arm to the double Popsicle sticks at the top of the A-frame on each side.
Attach rubber bands from the dowel at the bottom of the frame to the bottom of the arm on each side.
Test the trebuchet by pulling the top of the arm back and launching something from the soda cap bucket. Adjust as necessary to get the proper tension.