With the default Camera app, it's easy to use the Apple Watch as a remote shutter release. When you launch the Camera app on your Apple Watch, it opens the Camera app on your iPhone and offers a live preview and on-screen buttons to snap a shot or self-timed burst mode shots.
Thanks to a crafty Reddit user's discovery, you can also use the Apple Watch to take better selfies with third-party apps by tricking your iPhone into thinking you are pressing one of the volume buttons, which act as shutter-release buttons for some apps. To do so, open Snapchat or another compatible app to the camera-mode screen. Then on your Apple Watch, swipe up to access Glances and find the Music screen. Simply twist the Digital Crown and your iPhone will take a picture. You can also use the on-screen volume-up or -down button.
I found it worked with Camera+, Flickr and Snapchat. It also worked with Hyperlapse, but I found it was easier to control starting and stopping a time-lapse video with the on-screen volume controls; turning the Digital Crown stopped a video as soon as it started.