Essentially you are removing the systems RAM and reinstalling it, here are the steps for a MacBook. This works the same on other Macs, but removing the RAM is obviously different, so you’d need to understand how to do that. Here are instructions for MacBooks:
1) Shut down your computer
2) Remove the battery
4) Remove the L-bracket
5) Slide one of the levers (it doesn't matter which one) to the left. This will release the RAM
6) Gently wiggle the RAM card out and put it aside, you will put it back later
7) Replace the L-bracket and put the battery back in
8 ) Boot up the computer while holding COMMAND+OPTION+P+R (this resets the parameter ram)
9) Wait for the startup chime to sound 3 times
10) Release the keys and shutdown the machine once you reach the login screen
11) Remove Battery and L-Bracket, replace the RAM module and slide the lever back while pushing it in until it does not wiggle anymore
12) Replace the battery and L-Bracket
13) Now if you boot up the machine you should bypass the Mac firmware password.