To capture light trails, you need a long exposure so moving lights record as streaks. On an iPhone 6S, that usually means using a camera app that lets you control exposure time and ISO, since the stock camera may not offer the control you need.

Key points:

• Use a long shutter/exposure time, often several seconds.
• Keep ISO as low as practical to reduce noise.
• Adjust exposure so the scene isn’t over- or underexposed.
• Keep the phone completely still—use a tripod or very stable support.

Because smartphones have fixed apertures, exposure is mainly controlled with shutter time and ISO rather than aperture changes. A dedicated long-exposure app can help with this; one community answer specifically mentions a “Slow Shutter” app tutorial.

In short: mount the phone on a tripod, use a long-exposure app, choose a multi-second exposure, and experiment with ISO/exposure until the trails are bright but the rest of the scene is still properly exposed.