How I can save a screenshot of an iPhone application?
Press the home button and the sleep button simultaneously. Newer phones have the sleep,on the side instead of the top but save images the same. 

This will save a picture of the current iPhone screen to the "Photos" application. From there, you can either email it to anyone you want or sync it to your computer.



Assistive touch also works on newer iOS versions. 


How can I create iPhone screenshots without the lock button?


On iOS 11 screen recording might be if use for capturing transient events where your reflexes or the device itself isn’t responsive enough at the correct moment. 


How do I take a screenshot in iOS without hitting the Home Button and the Power Button?