How do I turn location on through another iPhone?

See If your iPhone, iPad, or iPod touch is lost or stolen https://support.apple.com/en-us/HT201472, this heading If Find My [device] isn't turned on on your missing device

"If you didn't turn on Find My [device] before your device was lost or stolen, it can't be used to locate your device.

But you can use these steps to help protect your data:

1. Change your Apple ID password. By changing your Apple ID password, you can prevent anyone from accessing your iCloud data or using other services (such as iMessage or iTunes) from your missing device.
2. Change the passwords for other internet accounts on your device. This can include email accounts, Facebook, or Twitter.
3. Report your lost or stolen device to local law enforcement. Law enforcement might request the serial number of your device. Find your device serial number.
4. Report your lost or stolen device to your wireless carrier. Your carrier can disable the account, preventing phone calls, texts, and data use.
5. Remove your lost or stolen device from your list of trusted devices.

Find My [device] is the only way that you can track or locate a lost or missing device. If Find My [device] isn't enabled on your device before it goes missing, there's no other Apple service that can find, track, or flag your device for you."

You have to have access to the device where you want to turn it on. It can't be done remotely.