Sign in to both devices with the same Apple ID
Connect both devices to Wi-Fi
Turn on iCloud for the data you want to sync
Go to Settings
Tap your name
Tap iCloud
Enable apps and services such as Photos, Contacts, Calendars, Notes, Reminders, and iCloud Drive
Turn on Messages in iCloud if you want texts synced
Turn on Photos in iCloud if you want photos synced
Use the same email accounts on both devices if needed
Turn on Bluetooth and Handoff if you want to continue tasks between devices
Go to Settings
Tap General
Tap AirPlay & Handoff
Turn on Handoff
Keep both devices updated to the latest iOS or iPadOS
Use the same app accounts on both devices for app-specific syncing
Back up both devices to iCloud regularly
Check that iCloud storage has enough space
Restart both devices if syncing does not begin
Sign out and sign back in to iCloud if syncing still fails