Before you start the Hard factory setting make
sure you back-up all important files (music,
videos, pictures etc.) on phone to the your SD
Card, better still you can back-up using “File
Manager” app, it can be downloaded from Play
Store . You can remove your Sim Card and SD
Card before you start or you can leave it there,
don’t worry hard reset won’t delete contacts on
your Sim Card and important files you kept on
your SD Card (External Memory Card).
Keys involve in the action
POWER KEY:
Is the key used in powering device on and off.
Power key is located at the side (left or right) of
most device or at the top of some device
VOLUME UP KEY:
Is the key used in increasing the volume of your
device
It is located at the side (left or right) of the
device usually with volume down key
VOLUME DOWN KEY:
Is the key used in lowering the volume of your
device
It is located at the side (left or right) of the
device usually with volume up key
Make sure the buttons are working, if one of the
buttons are not working correctly am sorry you
can’t perform the hard reset yourself, not until
you see an engineer.
If the button usually stuck shake your phone and
do it continuously until it is un-stuck.
HOME KEY:
It is usually located at the bottom of the screen
with a house logo
Now starting from the easiest and commonest,
carefully read the methods listed below to know
which best suit the type of Android or Tablet you
are using.
Method 1: Power off the phone, when it is fully
powered off press and hold the Volume Up and
the Volume Down keys at the same time, then
press and hold down the Power key (for about
20sec. at most) until a test screen shows, when
options appears you can release the keys.
Use the Volume Down key to navigate, in a case
like this you will be navigating down so press the
Volume Down key until it highlights factory reset
then press the Power key to select it.
Method 2: Power the phone off, press and hold
the Volume Down key and press then release the
Power key, still hold down the Volume Down key
for about 10-15sec.
When you see some options then you can let go
of the keys.
Press the Volume Down key to navigate down
through the options until it highlights reset
option, You will see factory reset, use the power
key to select it
Method 3: When the phone is powered off, press
and hold the Home key (usually located at the
bottom of the phone’s screen) continue to hold
the Home key then power the device on by
pressing the Power key, once the device recovery
screen appears release the Home key.
While on the recovery screen, press the Volume
Up and Volume Down keys both at the same
time.
You can release all the keys when you see the
system recovery screen, use Volume Down key to
scroll down through the options until it highlight
“wipe data/factory reset” then press the Power
key to select.
Select Yes-delete all user data and then press
OK
Method 4: This method is more suitable to
factory reset Samsong models like Samsong
Galaxy 3, S4, S5, Samsong Galaxy Note: Note II,
Note III etc.
When the device is powered off press and hold
Volume Up and Home key and the Power key all
at the same time.
The phone will vibrate or it display the Galaxy
logo, release the Power key but don’t release the
Volume Up and Home key yet, until the “Android
System Recovery” screen appears.
Use the volume button to scroll to highlight the
“Reboot System now” option and the hard reset
will be completed.
Note: Using hard reset will wipe out all phone’s
data so when your phone is turned up it will act
like it is just bought from the store (brand new),
you know… like “start from scratch” wiped out
There is no 100% assurance that this method
will work for all phones.
If you’ve tried one or all of these methods and it
didn’t work, go to settings on your device and do
normal factory settingsp
When you hard reset your device the first time
and nothings happens you might need to do it
again because some device may not understand
the action at first and it may think buttons are
mistakenly pressed so make sure you try it more
than once before you come “this is not working
for my device” kind of conclusion

Nice writeup.