To run bitcoin core testnet on Ubuntu 16.04

How can I run Bitcoin core testnet version on ubuntu 16.04 platform ? I am getting so many reference about how to run bitcoin core mainnet but it will take around 20 days to synchronize with the network as per my bandwidth and also will take my half of the HDD. So I opt for Testnet. Can someone explain me how to insatll bitcoin core testnet on Ubuntu 16.04? I would be grateful. Step by step explanation or reference would be much appreciated.