Sign in to the same Apple ID on both iPhone and iPad
On iPhone, go to Settings
Tap your name
Tap iCloud
Turn on Messages in iCloud
On iPad, go to Settings
Tap your name
Tap iCloud
Turn on Messages in iCloud
On iPhone, go to Settings
Tap Messages
Tap Text Message Forwarding
Turn on your iPad
On iPad, go to Settings
Tap Messages
Turn on iMessage
Make sure both devices are connected to Wi-Fi or cellular data
Keep both devices updated to the latest iOS or iPadOS version