Do the bulbs show up in the Home App? If so tap the + in the Home App and select Add Scene. If the Bulbs don't show in the Home App add them by tapping the + at the top of the screen and select add accessory, then manually enter the HomeKit Code into the App. Each Bulb HomeKit Code can be found on the Bulb. It's a series of number xxx-xx-xxx.
It's been awhile since I set up a LifX bulb but I recall during the setup it allowed you to add the bulb to HomeKit so you didn't have to go through any HomeKit set up. Either way you can manually add the bulbs by scanning or entering the HomeKit code.
Hope that helps?