How do I find my phone when its already dead?

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.

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