Firstly: 

If you have not resolved firmly to do ‘umrah during this trip, then you do not have to enter ihram from the miqaat, but if you decide to do it when you are in Makkah, then you should go out of the Haram, to al-Tan’eem or elsewhere, and enter ihram for ‘umrah. But if you have decided to do ‘umrah during this trip then you must enter ihram from the miqaat, because of the hadeeth of Ibn ‘Abbaas (may Allaah be pleased with him) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) defined the meeqaat of the people of Madeenah as Dhu’l-Hulayfah; that of the people of Shaam (Syria) as al-Juhfah; that of the people of Najd as Qarn al-Manaazil; and that of the people of Yemen as Yalamlam. And he said: “And these meeqaats are for the people at those very places, and besides them for those who come through those places with the intention of performing Hajj and 'Umrah; and whoever is living within these boundaries can enter ihraam from the place he sets out, and the people of Makkah can start from Makkah.”

Narrated by al-Bukhaari (1454) and Muslim (1181). 

Whoever passes by one of these miqaats, intending to do Hajj or ‘umrah has to enter ihraam from that point. If he lives within the boundary of the miqaats, then he should enter ihraam from where he is. If he is already in Makkah then he should enter ihraam for Hajj from where he is and enter ihraam for ‘umrah from any point outside the Haram (sanctuary), such as al-Tan’eem or ‘Arafah, because of the hadeeth of ‘Aa’ishah (may Allaah be pleased with her): when she wanted to do ‘umrah after her Hajj, the Prophet (peace and blessings of Allaah be upon him) told her brother to take her out of the Haram so that she could enter ihraam for ‘umrah from there. Agreed upon. 

See also question no. 32845. 

Secondly: 

If the person who wants to do Hajj or ‘umrah passes by the miqaat without intending to enter ihraam from there, then he has to go back to the miqaat that he passed, and enter ihraam from that point.  If he does not do that and he enters ihraam from the place where he is, then according to the majority of scholars he has to sacrifice a sheep in Makkah and distribute (the meat) to the poor of the sanctuary. 

The scholars of the Standing Committee were asked about someone who travelled in order to work during the Hajj season, and they stayed outside Makkah and did not enter ihraam from the miqaat. From where should they enter ihraam? 

They replied: 

With regard to ihraam for ‘umrah or Hajj, so long as you went there for work and passed by the miqaat, if anyone wants to enter ihraam he may do so from wherever he is within the miqaat boundary, because he entered with the intention of working. The Prophet (peace and blessings of Allaah be upon him) said, when mentioning the miqaats: “and whoever is living within these boundaries can enter ihraam from the place he sets out, and the people of Makkah can start from Makkah.” But if any of you had decided to do Hajj or ‘umrah when he passed the miqaat, then he has to go back to the miqaat and enter ihraam from there, because the Prophet (peace and blessings of Allaah be upon him) said, when he defined the miqaats: “And these meeqaats are for the people at those very places, and besides them for those who come thorough those places with the intention of performing Hajj and 'Umrah” End quote. 

Fataawa al-Lajnah al-Daa’imah (11/140, 141) 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said: 

If a person passes the miqaat without entering ihraam, one of two scenarios must apply.  

Either he intends to do Hajj or ‘umrah, in which case he has to go back to the miqaat and enter ihraam from there for whichever ritual he wants to do, Hajj or ‘umrah; if he does not do that then he has omitted one of the obligatory duties of the pilgrimage and according to the scholars he must offer a ransom (fidyah), a sacrifice to be slaughtered in Makkah and the meat distributed to the poor there. 

Or, if he passes it without intending to do Hajj or ‘umrah, then he does not have to do anything. End quote. 

Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (12/question no. 341). 

And Allaah knows best.