Open the web app
Open https://<the machine's address>:5400 in a browser on your home network.
The browser warns about the certificate: the server made it itself, because nothing on a home network can get a public one. Accept it for this address. See The certificate warning.
Create the administrator
The first visit creates the administrator. Add the rest of the household under Accounts: each person gets a temporary password and picks their own on first sign-in. See Accounts for the family.
Pair the first phone
In the web app, open Devices › Pair a phone and scan the QR code with Piktures. See Pair a phone.