I am not sure if the word "combine" is related to "Collection and concatenation" (Gama' and Kasr) جمع و قصر for the prayers.
so let's first explain what is collection and concatenation of prayers.
it means that in some circumstances you are allowed to pray the four rakaa' prayers like Zuhr, Asr and Isha' in a short version of 2 prayer however it should be concatenated to another prayer.
for example you can pray Zuhr (2 rakaa') followed by Asr (2 rakaa') at the time of either one of them.
and you can pray Maghrib (3 rakaa') followed by Ishaa (2 rakaa') at the time of either one of them.
when combining two prayers you shouldn't leave long period between two of them, as once you finish the prayer of the first one (Zuhr or Maghrib) start the Ikama call of the proceeding prayer (Maghrib or Ishaa).
When to combine prayers? During travel. to define traveling it means you travel for something that doesn't contradict with Islam so the travel can't be for the sake of doing something against Islam. the travel period should be equal to 3 days walking. (no matter how fast the modern method of transportation) a distance of 80 km is considered a travel so if you are traveling for a trip that is more that 80 km, you are allowed to combine prayers during the travel.
you actually combine prayer during the travel, not before you travel and after you reach your destination place you can still combine prayer for 3 more days.
in case you pray in group gamaa' you will not combine the prayer and pray as the residents.
in case you mean to combine that for any reason you missed a prayer and the proceeding prayer time has come. you do pray the one on-time then do the one you missed after it not before. for example if you missed the Maghrib prayer and Ishaa azan has raised you do pray Ishaa first then Maghrib.