You can't.
If you removed the phone from your Apple Account, that would have immediately cleared Activation Lock, letting the thief "make your phone their own". The phone might continue to appear in Find My for up to 30 days … but Activation Lock has been cleared already, and there is no way to remotely undo the removal.
My understanding is that if the thief leaves the phone turned off for 30 days, and then turns it back on and allows the phone to connect to the Internet, Activation Lock might automatically turn back on. But there is fat chance of the thief being so kind as to let that happen.
Likewise, if you fell for a "phishing" email and gave the thief your Apple ID and Apple ID password – allowing the thief to remove Activation Lock themselves – there is no way of turning it back on. (If you did give the thief your credentials, you should immediately take steps to re-secure your account. See: If you think your Apple Account has been compromised - Apple Support .)
The only thing you can do is to file a report with the police, and to contact your phone company (carrier) and ask them to blacklist the phone. If they do, the stolen phone will never be able to get cellular voice, message, or data service from any carrier who honors the blacklist. This will render it useless as a phone, although the criminal will still be able to reset it and use it as an "iPod touch".