How to load Atmega328p 8mhz board using board manager
Assuming that you have a 3.3V Atmega328P and are using an external 8MHz occilator:  

In the Arduino IDE click on Tools.
Scroll down until you see an option starting with the word Board:
Select the option called Arduino Pro or Pro Mini  

Click the Tools menu once again.
This time select the option starting with Processor:
Select the Atmega328P (3.3V, 8MHz)
Done!

Alternatively you can edit the boards.txt file located somewhere similar to C:\Program Files (x86)\Arduino\hardware\arduino\avr
Fuse bits can be calculated using one of these online tools:  


http://www.engbedded.com/fusecalc/  
http://eleccelerator.com/fusecalc/