Making clouds for science project
To form a cloud, generally speaking, pressure does not play a significant factor (with the exception of the effect on temperature).

If you want to prove that to yourself, go outside when it is foggy and measure the air pressure. Fog is a cloud, and pressure likely hasn't changed as much as it does in the vertical direction.

As far a forming a cloud, it is actually relatively easy. You only need to decrease the temperature to it's dewpoint. You may want relatively dirty air, so that the solute effect makes it easy to form drops. Silver iodide would be good for this.

For reference, the Kohler curve describes the saturation needed for cloud droplets to form.