How to take a Screenshot on iPhone and iPad?
Two buttons at the same time. The trick is releasing them straight away.
iPhone Screenshot Method
Taking a screenshot on an iPhone is also quite easy with a very simple method. Follow the steps below to take a screenshot of anything that you want. Open anything on your screen that you want to take a screenshot of. After opening it, you have to press Side button + Volume up buttons at the same time.
A white flash appears on the screen, and you can preview the screenshot. This screenshot can be edited as you want and will be saved on your iPhone.

iPad Screenshot Method
Taking a screenshot on an iPad is very easy by following simple steps. You can take a screenshot using the same method as you do on an iPhone. If your iPad has a Home button, you can press the Top button and the Home button simultaneously to take a screenshot.
How to Take a Screenshot on an iPad with no Home Button?
If there is no Home button on an iPad, you can still take a screenshot with alternate buttons. You need to press the Top button and either Volume button at the same time. If you want to take a screenshot with an Apple Pencil, you have to swipe up diagonally to capture the screen. These screenshots will be available in the Photos app.
If there is no Flash and no Screenshots
Press Sleep/Wake and then press immediately Home button then release. You screenshot will be captured and saved as a Photo in your gallery. Do not hold any button for long time because it will open trigger the emergency/power menu.
Which one is the Sleep/Wake button on iPhone?
It has different locations on different devices. You can find it either at the left or on top (iPad), at the top (e.g. iPod touch) or on the right side (e.g. iPhone 7, 8, X and newer).

Where Screenshots are saved on iOS?
You can find the screenshots in Photos App
on iOS.