From where should the pilgrim who is doing Hajj tamattu‘ enter ihram? Does he have to shave his head after stoning the Jamrah?

Firstly: 

Hajj tamattu‘ means entering ihram for ‘umrah only during the
months of Hajj – the months of Hajj are Shawwaal, Dhu’l-Qa‘dah and
Dhu’l-Hijjah – so when entering ihram for it, one should say: “Labbayk
‘umratan (Here I am for ‘umrah).” 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said,
describing ‘umrah of tammatu‘: 

When the pilgrim reaches the miqaat, she should do ghusl as
in the case of janaabah, and apply the best perfume he can find to his head
and beard, then put on the ihram garments, an izaar (lower garment) and rida’
(upper garment). It is preferable for them to be white and clean. Then he
should say: “Labbayk ‘umratan (Here I am for ‘umrah).” There is no
need for him to say that he is connecting it to Hajj, exiting ihram in
between, because he has already formed the intention in his heart that he is
going to perform Hajj. So he should say: “Labbayk ‘umratan,
Labbayka
Allaahumma labbayk, labbayka laa shareeka laka labbayk. Inna al-hamd
wa’l-ni’mata laka wa’l-mulk, laa shareeka lak 
(Here I am for
‘umrah.
Here I am, O Allaah, here I am. Here I am, You have no partner, here I am.
Verily all praise and blessings are Yours, and all sovereignty, You have no
partner).” Men should
raise their voices when saying this, and it should be understood that no
tree, rock or brick hears him but it will testify on the Day of Resurrection
that he said this Talbiyah. What the Talbiyah means is responding to Allah,
may He be glorified and exalted, because the word “labbayk” means: I am
responding to You. He should continue reciting the Talbiyah until he begins
the tawaaf of ‘umrah. When he reaches Makkah, he should do tawaaf and sa‘i
for ‘umrah, shave or cut his hair, and exit his ihram. Then on the day of
at-tarwiyah, which is the eighth day of Dhu’l-Hijjah, he should enter ihram
for Hajj only and do all the actions of Hajj. So the one who does tamattu‘
does a complete ‘umrah and a complete Hajj.

End quote from al-Liqa’ ash-Shahri. 

The one who is doing Hajj tamattu‘ enters ihram on the eighth
day of Dhu’l-Hijjah from the place where he is staying. When he enters ihram
for Hajj, he does the same as he did when he entered ihram for ‘umrah, doing
ghusl and putting on perfume. So he should form the intention to enter ihram
for Hajj and say “Labbayka Hajjan (here I am for Hajj)”, then recite
the well-known Talbiyah. 

The scholars of the Standing Committee for Issuing Fatwas
said: 

With regard to the one who enters ihram for ‘umrah from the
miqaat, intending to follow it with Hajj tamattu‘, when he has done ‘umrah
he should exit ihram. There is nothing wrong with him going to Jeddah or
elsewhere, then entering ihram for Hajj from the place where he is staying
after doing ‘umrah.

End quote from Fataawa al-Lajnah ad-Daa’imah, vol. 2
(10/76-77) 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

What is prescribed for the pilgrim who is not in ihram is to
enter ihram on the day of at-Tarwiyah from the place where he is, whether he
is inside Makkah or outside, or in Mina, because the Prophet (blessings and
peace of Allah be upon him) instructed those who had exited ihram after
‘umrah to enter ihram for Hajj on the day of at-Tarwiyah from the places
where they were staying.

End quote from Majmoo‘ Fataawa Ibn Baaz (16/140)

Secondly: 

As you are going to do ‘umrah on the seventh day (of
Dhu’l-Hijjah), you should cut your hair after finishing ‘umrah, and not
shave your head, so that some of your hair will remain for you to shave it
off during Hajj. The Prophet (blessings and peace of Allah be upon him)
instructed his companions, during the Farewell Pilgrimage, to cut their hair
following ‘umrah, because their arrival was on the morning of the fourth day
of Dhu’l-Hijjah. This indicates that cutting the hair in this situation is
preferable to shaving, and it is better so that the head can be shaved
during Hajj. For more information please see fatwa no.
31822 

Thirdly: 

The actions on the Day of Sacrifice, for the pilgrim who is
doing tamattu‘ are, in order: stoning Jamrat al-‘Aqabah, offering the
sacrifice, shaving the head or cutting the hair, tawaaf al-ifaadah and sa‘i.
This is the best way, because this is what the Messenger (blessings and
peace of Allah be upon him) did. For more information, please see the answer
to question no. 106594 

If the pilgrim does some of them earlier and others later, or
he does some of them after the Day of Sacrifice, there is nothing wrong with
that. 

The scholars are unanimously agreed that it is permissible to
delay shaving the head until the last of the days of at-tashreeq, but if the
pilgrim delays it any longer than that, then he must offer a compensatory
sacrifice, according to some scholars. See al-Mawsoo‘ah al-Fiqhiyyah
(10/12-13) 

Try to do your Hajj with one of the scholars or seekers of
knowledge, so that you can ask him whatever you need to know about Hajj and
‘umrah. 

And Allah knows best.