Step 1: This We Are Going to Use...
pastel paper or thin cardboard sheet.
pencil and scale.
duck and transparent tape.
air pump ( hand).
a curious mind etc.
Step 2: Make Its Fin (not a Nice Word to Use Here).
Step 3: Make Its Head, So It Becomes Aerodynamic.
cut out a sector from the circle in order to make a cone of it. Make the base of the cone accurately the size of bottles base. Add tape to the cone, to make it rigid.
Step 4: Join the Fins and the Head Cone to the Bottle Correctly.
Mark 120° on the surface of the bottle. Join the fins on the marks correctly. So it should not vibrate or did balance while flight.
Step 5: Add Some Weight on the Neck of Bottle.
Step 6: Making the Thrusting System.
Step 7: Make Up a Launching Pad.
Step 8: Take Off.
Pump some air. Do until the rocket takes off. If you want to make its range more, you can add a pin on the neck of bottle which will able to control the pressure of inside. Just pull the pin safely and your rocket will zoom even higher in sky. You can take help of elders in launching. Your area should be clear of debris, houses, wires and any other things that can cause trouble.