https://learn.adafruit.com/usbtinyisp/faq
How do I program a bootloader onto an Arduino?
Put a fresh AVR chip (such as an Atmega328) into the Arduino in the correct orientation
Remove the jumper from the USBtinyISP
Plug in the USBtiny to USB
Plug the Arduino into DC or USB so it is powered
Plug the 6 pin cable from the USBtinyISP into the Arduino so that pin 1 mark is lined up with the red wire on the cable
Start up Arduino IDE
Select the chip/Arduino you are using in the Tools->Board menu
Do not select a COM/Serial port
Select Tools->Burn Bootloader->w/USBtinyISP
The USBtinyISP red LED should light up. It will take a minute or two to program the chip
When it is done, the IDE will tell you it has completed and the red LED will be off.