Note: You need to turn on the screenshot setting first. To do this,
-> Open the Apple Watch app on your iPhone.
-> Tap the My Watch tab and then General.
-> Scroll down, and turn on Enable Screenshots.
Press and hold the Digital Crown and then press the Side Button = The screen flashes. This action captures and saves the entire screen to the Photos app on your iPhone.
Open the Photos app on your iPhone, tap the Albums tab, then tap Screenshots.
The screenshot shortcut on the Apple Watch is also the shortcut to pause and resume workouts. So if you use the Workout app and you enable screenshots, you'll take a screenshot every time you pause or resume a workout.