how do i add a device to my apple id

Sign into iCloud on that device with your AppleID.

You need to use it when you set up your new device. If you need to start over, use What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touchhttps://support.apple.com/en-us/HT201351. And, then use Set up your iPhone, iPad, or iPod touch https://support.apple.com/en-us/HT202033