How does one who entered Mecca without ihram, while he is in it, assume ihram for 'Umrah?

If you went to Mecca for Umrah and intended to perform Umrah, you must enter into a state of ihram from the Miqat you pass through. If you now return to the Miqat and enter ihram from there, your Umrah is valid and nothing is incumbent upon you. However, if you entered ihram from Mecca, you are required to offer a sacrifice (a sheep) to be slaughtered in the Haram and distributed among its poor, and your Umrah is valid.