There was a previous combo.. power button and windows logo; press them at the same time. This was for devices on WP 8.0 I think..
The current screenshot combo is.. power button and volume up button. Holding them down for less than 5 seconds should already be sufficient. You should hear a camera shutter sound as confirmation and then at the top of your screen you'll see a text that says "saving your screenshot".
All screenshots can be found in the photos app. You might have to manually select the screenshot folder