Once I had downloaded the Trainline app to my Android phone and then logged into my account, the tickets were all right there for me to access. It was very easy.
However, a cautionary tale: I lost my phone in the Madrid airport at the end of the trip. It happens. I mistrust the phone for holding holding tickets and boarding passes, etc. for just this reason. I printed everything out in hard copy format before we left home and used them as the primary tickets. The phone was just a back-up.