Use a camera to take a picture of the HomeKit code or QR code on your Hue light. Alternately you can also choose to enter the code manually.
If you have an iPhone 7 or later, and see the the below symbol instead, just hold your phone next to the accessory for a few moments.
You'll be asked to enter the details about the light, such as what room the device is in, the name you want to give it. Enter this information so it's easier to find in the future.
Tap Done when you've got all the information entered.