If you buy an Advance ticket it is only £19.20 as against £43.10 for a flexible ticket.
But then you have no certainty over what time your flight will land, or how long immigration will take. So sensible to buy the £43.10 ticket.
You can equally just buy the £43.10 ticket at the station on arrival. Whichever way it is valid on any train that day.
The railway station is within the South Terminal. If you land at North Terminal take the free inter terminal train to South.
Present your ticket to the ticket gates at the station and pass through. If a 'phone ticket lay the phone flat on the gate infra red reader. If a paper ticket bought on the day put it in the gate slot and wait for it to come back out when the gate will open. The overhead signs will tell you which platform you need- there are 7 platforms but in normal circumstances all Cambridge trains leave from Platform 4- do check in case of any alterations.
The guard will probably re-check the ticket on the train.