You buy the ticket in the normal way online.
Then shortly afterwards, the function of 'Download NFC pass' appears on your order history. To be able to download the ticket to your phone, you'll need a Google account and the Google Pay app, or you could use the Apple equivalent if you have an Iphone. You click on Download NFC pass, and it takes you to the links to log into either your Apple account, or your Google. Once you do that, it downloads it to your account, and is then visible on your phone.
If I were you, I'd get the Google/Apple Pay ASAP, and put your debit card on there and make a couple of contactless payments, just so you can get used to using it. Your phone will also need to have some sort of security, ie: pin code or fingerprint to get into it, as this is a requirement of using contactless technology.