How do I take a screen shot?

iPhone 7, Home and side, simultaneously. (iPhone 7 and 7 Plus, no iPhone 7s.)

iPhone 4s, Home and top, simultaneously.

See this article > Take a screenshot on your iPhone - Apple Support

Maybe also > Identify your iPhone model - Apple Support

iphone 4S I believe you hold down the top button then immediately double tap the Home button

If I'm wrong someone will come in.

Sorry, small senior moment.