How do I find an iPhone that is turned off ?

If you enabled the Find My option on the phone.....before.....it was lost, you still might be able to locate the phone even if it has been powered off. You don't have to do anything to make this happen.

The reason for this is that the phone still sends out a brief intermittent signal that can be picked up by other iPhones in close vicinity to the lost phone......as long as the battery holds out.

Follow the instructions in this Apple Support document:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Now, if the battery on the phone has run down, or the phone cannot connect to other iPhones that might be close to the lost phone, then the phone's location cannot be displayed on the Find My map.

If that is the case, all that you can do is keep checking a few times each day to see if the status of the phone has changed.