How do I pair pencil 1 to new IPad Air?

Plug the Apple Pencil into the Charging port of the iPad. A pairing notice should appear.

See here: Use Apple Pencil with your iPad or iPad Pro - Apple Support

Check the compatibility here:--> Apple Pencil compatibility - Apple Support

Your iPad Air must be a 3rd generation model (but you didn't specify that).

Regards

Giulio