A couple of ways, really, as long as people know what you mean!
This equation isn't balanced, but it doesn't really need to be for this question..
Some ways you can say this are
"Hydrogen gas burns in air to produce water vapor"
"Hydrogen gas and oxygen gas combine to form water vapor"
"Hydrogen gas reacts with oxygen gas to produce water vapor"
"Water vapor is formed from when hydrogen burns in air"
"Water vapor forms when hydrogen gas and oxygen gas combine chemically"
"Water vapor is formed when hydrogen gas reacts with oxygen in the air"
If we go ahead and balance the equation:
then we could also say in words:
Two molecules of hydrogen gas plus one molecule of oxygen gas produce two molecules of water vapor.
Two moles of hydrogen gas plus one mole of oxygen gas produce two moles of water vapor.