How do I turn on "Find My" without my actual phone? It is dead and lost.

Sorry, there is no option to turn on Find My remotely.

If you are certain Find My wasn't setup before the phone was lost, see: If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support

If there is a possibility that it was on, see the following Apple articles for information about lost or stolen iPhones or iPads: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support and

Use the Find My app to locate a missing device or item - Apple Support

Bob Timmons wrote:

...

To appear on the Find My map, the iPhone must have a good battery charge and a good Internet connection.

Apple recently implemented some magic (to me) that changes things a bit:

See the following from Use the Find My app to locate a missing device or item - Apple Support

If you turn on Find My network, you can locate your device or item even if it's not connected to a Wi-Fi or cellular network. The Find My network is an encrypted anonymous network of hundreds of millions of Apple devices that can help you locate your device or item. 

and

• If you're using the Find My app, you can't see your device's location if more than seven days have passed since its last location was sent to Apple.
• If you're using iCloud.com/find, you can't see your device's location if it's powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple.

Sorry, this is not possible. Find My must be turned on at the iPhone manually...before the phone is lost. It cannot be turned on remotely.

However, even if you have turned Find My on at the iPhone, with the battery run down it would not appear on a Find My map at all.

To appear on the Find My map, the iPhone must have a good battery charge and a good Internet connection.

I've tried the "magic" and it is not there for me. How a phone can broadcast information about its location on a dead battery has me puzzled. The best it might do is broadcast info when the battery has really run down, but is still barely operating. Maybe the battery in my old phone was "most sincerely dead" when I tried this.

I believe that is what it does -- the last location before the battery died is displayed for 7 days in Find My.

I believe the ability to locate without Wi-Fi or Cellular probably also requires at least some battery power.

It’s too late.

That’s like asking how to back up the iPhone after it’s broken. Or how to purchase auto insurance after a car crash.