Sign in to the same Apple ID on both devices
Turn on Wi-Fi and Bluetooth on both devices
On iPhone, go to Settings
Tap your name at the top
Make sure iCloud services you want are enabled
On iPad, go to Settings
Tap your name at the top
Sign in with the same Apple ID used on iPhone
Turn on iCloud syncing for Contacts, Photos, Notes, Calendar, and other apps you want
Enable Handoff on both devices in Settings
Turn on AirDrop on both devices if you want to share files
Use iCloud Drive to access the same files on both devices
Turn on Messages in iCloud if you want texts synced
Turn on FaceTime with the same Apple ID on both devices
Use the same Wi-Fi network for easier syncing
Keep both devices updated to the latest iOS or iPadOS version