How Do I Set The Orientation Lock On My iPhone?
The default method for setting orientation lock on an iPhone or iPad is to double-tap the Home button to bring up the App Switcher. From the App Switcher, swipe to the right and you will see music controls and a button with a circular arrow. Push that button to set orientation lock.
On the iPad, you can use the side switch to toggle lock rotation but it's not the default option. To enable it, go to the Settings app to General. Scroll down to "Use Side Switch to" and select Lock Rotation.