If you buy the version with the chime, it comes with a transformer too. You need to put this in a suitable enclosure (e.g. ESE2) and supply from a suitable source (e.g. 3A SFCU) if you don't have an existing bell transformer that does 24V (unlikely, most are about 8V). Bell wire from the transformer to the doorbell.
Once power is applied, set up the app on the customer's phone (they will need to register etc). The doorbell generates its own wifi network, the phone (via the app) connects to this, the two are paired, the app then tells the doorbell about the house wifi (gives it the house wifi password), doorbell and phone then switch to using that.
Plug in chime. Tell app you have a chime. It should find it (again, the chime generates its own wifi network which the phone app links to, then tells the chime about the house wifi, then they both switch to that).
A right faff, particularly when you get to the point of installing the chime and it fails to light up or in fact do anything, and you end up talking to Chuck in the USA for half an hour while they sort out shipping a replacement so you don't have to uninstall the doorbell and transformer and get the customer to return them to John Lewis.
I always, always do these jobs on a "by the hour" basis as you never know what little gremlins of joy you're going to discover.