How to create deuterium oxide

Fig 1.Production of deuterium oxide (heavy water) - Girdler sulfide process

As semi heavy water, HDO occurs naturally on Earth in regular water at a proportion of 1 part per 3200, it may be separated from regular water by distillation or electrolysis and also by various chemical exchange processes, all of which exploit a kinetic isotope effect. In short, the difference in mass between the two hydrogen isotopes translates into a difference in the zero-point energy and thus into a slight difference in the speed at which the reaction proceeds. Once HDO becomes a significant fraction of the water, deuterium oxide (heavy water) will become more prevalent as well as water molecules trade hydrogen atoms very frequently. To produce pure deuterium oxide by distillation or electrolysis requires a large cascade of stills or electrolysis chambers, and consumes large amounts of power, so the chemical methods are generally preferred. The most important chemical method is the Girdler Sulfide process.

Source:

https://en.wikipedia.org/wiki/Heavy_water#Production