Sign in to the same Apple ID on both devices
Connect both devices to Wi-Fi
Turn on iCloud sync for the content you want to share
On iPhone, go to Settings
Tap your name
Tap iCloud
Turn on Photos, Contacts, Calendars, Notes, Reminders, and other apps you want synced
On iPad, go to Settings
Tap your name
Tap iCloud
Turn on the same iCloud options
Turn on iCloud Drive if you want files synced
Turn on Messages in iCloud if you want texts synced
Turn on Keychain if you want passwords synced
Use AirDrop to send individual files between devices
Use Finder on Mac or iTunes on Windows to back up and restore if needed
Keep both devices updated to the latest iOS or iPadOS
Make sure both devices have enough iCloud storage available