1. For a start you have to turn off your device.
2. After that, you will have to press and hold volume down + power + lock key at the same time.
3. Wait untill device will vibrate, then release the keys when the factory hard reset screen appears. It should look like this:
4. Using volume keys choose wipe data/factory reset and press the power/lock key to confirm.
5. Congratulations, just wait for the process to be over and you are done.