Screen shot

[Ray1]

Pressing the power button and volume down button at the same time  with a quick press

[Georgine]

Hi, Welcome to the Samsung Members Community Canada. Use the following methods to capture a screenshot. Captured screenshots will be saved in Gallery.
• Key capture: Press the Volume Down key and the Power key simultaneously.
• Swipe capture: Swipe your hand to the left or right across the screen.
• It is not possible to capture a screenshot while using some apps and features.
• If capturing a screenshot by swiping is not activated, launch the Settings app, tap
Advanced features → Motions and gestures, and then tap the Palm swipe to
capture switch to activate it.

[Cassidy]

Below are the methods to take a screenshot:
Method 1: Volume + Power key combination
> Open up whatever you want to take a screenshot of on the display, be it an app, game, webpage, picture, etc.
> Simply press and hold the power button and the volume down button simultaneously until the camera shutter sound is played and you see an animation. This provides you with both an audio and visual cue to let you know that the device has captured a screenshot.
Method 2: Palm Swipe
> You can take a screenshot on the Galaxy device using a simple gesture as well. Just swipe the screen with the edge of your hand from right to left or left to right to capture a screenshot. You’ll get the aforementioned audio and visual cues as well.
> This gesture can be enabled from the Settings » Advanced features menu. Navigate to the menu and toggle the “Palm swipe to capture” option to on.
Method 3: Bixby
> This is perhaps the most simple way of taking a screenshot on the Galaxy S9 and the Galaxy S9+. Wake up Bixby by saying “Hi, Bixby” to activate the assistant by pressing the dedicated Bixby key.
> Simply ask Bixby to “Take a screenshot” and it will happily oblige.