Sign in to both devices with the same Apple ID
Connect both devices to Wi-Fi
Turn on iCloud sync for the data you want to share
On iPhone, go to Settings > [your name] > iCloud
On iPad, go to Settings > [your name] > iCloud
Enable syncing for Contacts, Photos, Notes, Calendar, Reminders, and other needed apps
Turn on iCloud Drive if you want to sync files
Turn on Messages in iCloud if you want iMessages on both devices
Turn on Keychain if you want passwords synced
Turn on automatic app downloads in App Store settings if needed
Use the same app accounts on both devices for third-party apps
Keep both devices updated to the latest iOS or iPadOS
Back up both devices to iCloud regularly
Use AirDrop for quick one-time transfers
Use Finder on Mac or iTunes on Windows for manual syncing if needed