The easiest and simplest way is to do as always, press the power button and volume down button at the same time,considering that the power button is on the back of the terminal. If you are used to the previous LG X mach you will be comfortable but when you come from a side button is somewhat more complicated.
Just make sure to take screenshots without notification by that there is nothing uglier. The screenshots you take with this method are saved to the Pictures folder inside your gallery for you to view later, if you want to share the capture when you click on the capture and ready.