first you’d have to purify water by distilling it and then putting it through an R.O. with a regular water filter as a pre-filter.

Then once you have that completely pure mix of hydrogen and oxygen you’d need to do electrolysis on the water. But that initial purifying process would have to be done very strictly, because if there’s even the smallest amount of salt or even chlorine in that water, or any other minerals then you could have a mixture of some very dangerous elements in the air around the water