Use Camera Remote on Apple Watch — Apple Support By Apple Support

You can use your Apple Watch to snap a photo with your iPhone camera. Here's how. First, open the Camera Remote app on your Apple Watch and position your iPhone. You can try balancing it against an object or using a tripod. Now frame your shot, using your Apple Watch as the viewfinder. By default, the Shutter button has a three-second timer; but you can also tap the More button for additional settings, like turning off the timer, switching between the rear and front camera, and more.

Tap Done to go back to the viewfinder, and then tap the Shutter button to take the picture. ♪ Great shot! ♪.

Source : Apple Support