Open Control Center by swiping up from the bottom of the screen (or swiping down from the top right on iPhone X and newer). Tap the Lock Rotation / Lock Orientation button a second time, so that the ...
Pull down Control Center from the top right. Make sure your iPad is in the orientation you’d like it to be locked in. Underneath the system functions (Airplane Mode, Wi-Fi, Bluetooth, etc)., tap on ...