How do I add a device to my current apple id?

Each device has its own set up instructions, which include adding it to your current Apple ID.

IPhone or iPad: Set up your iPhone, iPad, or iPod touch - Apple Support

Apple Watch: Set up your Apple Watch - Apple Support

Short answer: sign-in to your AppleID w/ that device as Carol posted.

It is “then” a Trusted Device on your AppleID.

This is why you wan’t to make sure anybody else’s “stuff” has been scrubbed off the device beforehand.

See the Add section.

Apple ID device list -Check to see where you‘re signed in.