As the Newton is the Standard International unit of force, you want to make a force meter. Before starting, you need to remember that one Newton is the force required to cause a mass of one kilogram to accelerate at a rate of one meter per second squared, in the absence of other force-producing effects. In general, force (F) in Newton, mass (m) in kilograms, and acceleration (a) in meters per second squared are related by the well known formula in Physics:

The experiment uses a Two-Hole Stopper in a bottle with two pieces of tubing through the holes. In order to get measurements, the bottle has to be sealed up.

Step 1. Half fill the bottle with water.
Step 2. Tape the longer tubing to a wall or board to keep it vertical. Tape a yardstick or meter stick along-side the piece of tubing.
Step 3. Inflate a balloon. Carefully wrap the neck around the end of the loose piece of tubing and hold it in place with your fingers. Make sure no air is escaping between the neck of the balloon and the tubing.

If the arrangement is working correctly, water should rise up in the vertical piece of tubing when you release the air in the balloon.

Once you make this measuring device, you will know the water mass because you filled it, and you will measure its displacement, and time. From these measurements, you can get the force in Newton from the formula F = m a.