We have quite a few threads about this subject. I have collected some of them and arranged them by the OS which is primarily discussed in the thread. That is because the exact procedure depends on the OS involved. What's more, since you often need to interact with the boot process, the procedure can vary somewhat based on your exact hardware.
Despite all of the that, there is considerable commonality in the procedures so you should take the time to go through all of the threads. You may get an idea from one of the threads.
You may still need help after you through these threads. If so, help us help you. Give us the details about which OS you have and the hardware you run it on.