Tap to Lock

"I will teach you guys a simple way on how to lock your Mi phone without pressing the Power button instead you will just simply click the Lock icon."

The first thing to do, Go to Home screen and then click Menu key button> Then click "Edit Home Screen." > click "Widgets." > and then click "Toggles." > Swipe to right, find the Lock icon there. and just simply click it and then there's Lock icon will place on the home screen. Done!

See my Screenshots: