I'll be explaining on Samsung Galaxy Ace 3
Pre-Requirements:
Samsung USB Drivers (Generic)
Odin
And: your phone's model-specific firmware
You can find it here: Sammobile website
For Example: my phone is Samsung Galaxy Ace 3: GT-S7270
So i'll search for the firmware file of GT-S7270, not any other variant!
Although GT-S7272 and S7275 are variants of Ace 3, i can't use them!
It will be very risky, THE SAME APPLIES TO ANY OTHER MODEL
Steps:
1- Install USB Drivers, Only if you haven't installed them before
2- Extract Odin.zip
3- Extract the large zip that containts your firmware
4- Open Odin.exe and make sure "Auto reboot" & "F. Reset Time" are checked
5- Select PDA, then select the large .md5 file of your firmware
6- Wait
7- Turn off your phone completely
8- Turn it on into download mode: probably using the key combination: Volume down + Home + Power buttons
9- Press volume up to confirm, and plug the phone in, odin should detect your phone
10- Now go ahead and click start
11- You should be done after the process finishes