my iphone has been stolen and find my app is showing it offline, how do I find its last location?

In order to see the last location, "Send Last Location" would have to have been turned on in Settings > [username] > Find My > Find My iPhone > Send Last Location in the device before it was lost. The description in Settings for that is "Automatically send the location of this device to Apple when the battery is critically low." The location would not be available if the device is offline because someone turned it off or because it doesn't have an internet connection, but only if it is off because the battery was low. The additional requirement is that the location is only shown for 24 hours. If all of those conditions are met you would see the last location in Find My. There is some information on that in If Find My is offline or not working - Apple Support