What to Do If You Miss Part of Tarawih?
Question

If I enter the mosque late for `Isha’ prayer and I missed the `Isha’ prayer in congregation and I pray it on my own, and I am late in joining the Tarawih prayers because of doing `Isha’, and I miss two Rak`ahs of Tarawih, how should I do these two Rak`ahs? Alone or what?
Praise be to Allah.Can you pray `Isha’ behind someone praying Tarawih?
If you miss `Isha’ and arrive when the Imam is praying Tarawih, it is better for you to join him with the intention of praying `Isha’ . When he says the Salam, you should complete your prayer and not pray alone or with another congregation, so that there will not be two congregations at the same time, which may lead to distraction due to the intermingling of voices. 
What to Do If You Miss Part of Tarawih?
As for what you have missed of Tarawih , if you wish you may do it, so you may pray two Rak`ahs when the Imam prays Witr, then pray what you missed, and then pray Witr.  
What is meant by praying two Rak`ahs when the Imam prays Witr is that you do not say Salam with him when he ends his Witr, rather you should stand up and pray another Rak`ah, then say the Salam. 
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked: If I join the people when they are praying Tarawih and I have missed part of it, should I make up what I have missed after Witr or what should I do? 
He replied: 
“Do not make up what you have missed after Witr , rather if you want to make up what you have missed, then pray two Rak`ahs when the Imam prays Witr, then offer the prayers you missed, then pray Witr. 
Here there is an issue to which attention should be drawn: If you arrive when the Imam is praying Tarawih and you have not prayed `Isha’ yet, what should you do?  Should you pray `Isha’ on your own or join the Imam in Tarawih with the intention of praying `Isha’? 
The answer is that you should join the Imam in Tarawih with the intention of praying `Isha’. When the Imam says Salam from Tarawih, then stand up and make up the rest of `Isha’. Imam Ahmad (may Allah have mercy on him) mentioned the exact same issue, and this was also the view favoured by Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him), and this is the correct view, because the correct view is that it is permissible to offer an obligatory prayer in congregation behind an Imam who is offering an optional prayer , based on the evidence of the Hadith of Mu`adh ibn Jabal (may Allah be pleased with him) who said that he used to pray `Isha’ with the Prophet (peace and blessings of Allah be upon him), then he would go back to his people and lead them in offering that prayer – it was optional for him and obligatory for them.” (Al-Liqa’ Ash-Shahri) 
It is better for you to make it up in congregation if you can, but if that is not possible then there is nothing wrong with you making it up on your own. 
And Allah knows best.