Either open Control Center and tap the Low Power Mode icon, or open the Settings app, tap Battery, and tap the switch labeled "Low Power Mode".

Jump to: Method B: Enable through the Settings app

Method A: Enable through Control Center

NOTE: You must add the Low Power Mode icon to the Control Center in order to use this method.

A1) Open Control Center.

A2) Tap the Low Power Mode icon.

Method B: Enable through the Settings app

B1) Open the Settings app.

B2) Tap Battery.

B3) Tap on the switch next to "Low Power Mode" to enable it.

Outcome

Your device is in Low Power Mode, which is indicated by the yellow battery status bar.