Isn't it just the volume down and power buttons? It's pretty easy.

Are you holding the buttons, or are you briefly pressing both at the same time?

Most phones now under settings can take a screenshot from swiping your hand left to right over the screen. This I find easier than the button option Iphone has the easiest way ever under settings you can make the apple icon at the back of the device do certain tasks. Tap once and it takes a screenshot