- Original Kindle, Kindle 2, Kindle DX, Kindle Keyboard - Press and hold Alt-Shift-G on the keyboard.
- Kindle 4 (the model with neither a touchscreen or a keyboard) - Press and hold the home button and the keyboard button.
- Kindle Touch - Press and hold the home button, and then tap the screen.
- Kindle Paperwhite, Kindle (2014) - These models don't have any buttons on the front, so Amazon had to get a little inventive when creating the trigger for the screenshot. To take a screenshot on the Kindle Paperwhite or the new Kindle, simultaneously press two opposite corners of the screen (upper left and lower right, for example).
- Kindle Voyage - I don't yet have any info on how to take a screenshot on the Voyage, but I bet the trick from the Paperwhite will work. (If that doesn't work, leave a comment and I'll help you find the answer.)
The screenshots are saved in different locations on the various models, and you'll need to connect your Kindle to a computer and transfer them over USB. Generally you'll find the screenshots in either the main folder or the documents folder. If you don't see them in either location, you probably didn't quite get the buttons presses in the right order.
P.S. If you're looking for instructions on how to take a screen shot on a Kindle Fire (or, as Amazon now calls them, a Fire tablet), here is what I generally try with Android devices.
Press and hold one of the volume buttons and the power button. If that doesn't work, try the other volume button and the power button. (This works on most Android devices, including a couple of the Fire tablets I have owned.)
And if that doesn't work, let me know. There are other possile combinations to try.