ANSWER:
You pray thuhur 2 rak’ah and then you offer salam. After that you pray Asr prayer 2 rak’ah and after that you offer salam.
You can do all of that depending on your convenience. The same goes for Maghrib and Isha, however maghrib is not shortened while Isha is. There are two types of combining, one is to pray it at the time of the first prayer and the second is to pray it on the time of the second prayer, whether in the beginning, middle or end.