Sign in to both devices with the same Apple ID
Connect both devices to Wi-Fi
Turn on iCloud on both devices
Go to Settings
Tap your name
Tap iCloud
Turn on the data you want to sync
Turn on Photos
Turn on Contacts
Turn on Calendars
Turn on Notes
Turn on Reminders
Turn on Safari
Turn on iCloud Drive
Turn on Messages in iCloud
Turn on Keychain if needed
Turn on Bluetooth if you want device pairing features
Enable Handoff on both 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
Back up both devices to iCloud regularly
Use Finder or iTunes on a computer for manual syncing if needed