Lost phone

Follow these steps if your phone is lost: https://support.apple.com/en-us/HT201472

Assuming you had Find My already enabled on that lost phone, you can try to track it at https://www.icloud.com/ Log in with your AppleID using any web browser on any platform to use the find my device app there to try and track it.

If you’re asking how to enable Find My remotely, that cannot be done. Find My can only be enabled when the device is physically in your hands.