Assistive Touch is in Settings - General - Accessibility. If you enable it, a transparent button will appear somewhere on the screen. It's not possible to post a screenshot of that button. Mine appeared here when I enabled Assistive Touch:
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When you tap the button, you'll get this:
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Select "Device", and "More" on the next page:
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Then you'll find "Screenshot".
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Tap it to take a picture of the screen.
It's not as easy as just pressing Home and Power button, though.