First, Assistive Touch needs to be turned on. It can be turned on in iPhone Settings, or with triple-click home button.

In Settings -> General, go to 'Accessibility' Then go to 'Triple-click Home' and select 'Assistive Touch'.

Make sure your iPhone's rotation lock is turned on.

Press the Home button three times (triple-click), black box with white circle, the Assistive Touch button, will appear as shown in the pic.

Pressing the Assistive Touch button will bring up a menu. Select 'Device'.

Select 'Rotate Screen'.

Select 'Left' or 'Right' depending on your desire.

Voila! Screen is locked in landscape mode. Triple-click again to dismiss the floating button.

How to lock landscape mode for iphone

It is easy to lock rotation in portrait mode, but difficult in landscape mode, screen will rotate to portrait mode when rotation lock is enabled. Here's how.