Use a pencil sharpener to partly sharpen the ends of the axles and use them to pierce a hole in the foam chassis by holding firmly then pushing and twisting into position using the large cut-outs to help keep you lined up.
Try to get the axles lined up properly or your vehicle will go in a circle!

When making the hole for the rear axle be careful to position the hole low down in chassis or the wheels when fitted will not touch the ground!

The wheels are held on using small plastic push-on axle clips made from plastic tubing.

The lightweight foam wheels have a plastic bearing inserted before  fitting to the axle then securing with the axle clip.

A standard party balloon is attached to a length of plastic tubing using an elastic band.

The tube is then fitted to a block of foam, the tail-piece, which fits into the hole in the chassis at the rear.

It takes a little practice to blow up the balloon and place the vehicle on the ground ready to go.

Try to have your finger ready to gently block the hole so as to stop the air escaping but don’t press too hard or you’ll push the tube back through the hole in the foam tail-piece.

Try using tubes of different thickness and see how this affects the speed of the balloon powered vehicle.