How to turn off landscape mode on iPhone 4S?

It's so annoying when I'm in bed using my phone sideways and it always goes to landscape. How to turn that off?

Press the home button twice, when the icons pop up from the bottom, swipe to the left. Now you should see some options that let you adjust brightness, sound, and a circle with an arrow which means portrait orientation, click on that circle and it will lock the phone into portrait mode

Double Tap Home button
Scroll to left until by the music controls
See button with arrow, tap that
Locks phone in portrait mode

Repeat to take it off again.