How do I find my phone when its already dead?

You physically go look for it where you think you lost it. There is no other way.

There is not much else you can do if your iPhone is dead other than put it in Lost Mode according to step 3 in the "If Find My [device] is enabled on your missing device" section of:

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

That way if someone finds your iPhone and charges it, they can get a message to contact you.