TAKING SCREENCAPS ON MAC
To take a picture of the screen on a mac, you simply need to hold Shift > Control > Command > 3 on your keyboard . This will memorize it, so that you make paste it later with Command > V.

ON WINDOWS
To take a picture of the screen on Windows computers, you just need to press the Print Screen button on the top row of keys on your keyboard. To paste, use Control > V.

I hope you find that both of these work effectively. If not, feel free to contact me or ask me a question.