You will also need software to back up your drives without it having to be installed. You will have to do some research as they change all the time. The two I have used most is Clonezilla and Ghost. Clonezilla is free for personal use. Best to read the manual before using the software. Even better would be to get a professional to help you out the first time.
The procedure is to back up the original hard drive to a second drive. Replace the original drive with the first backup drive. Now you can make any changes you want without losing the pristine image if you make a mistake. One you have the backup drive configured with all the needed software, you will want to make a second backup. so you have a Grandfather, father and son images. You are backed up twice so to speak, If you have purchased duplicate machines, you can image them from the backup without having to manually setup each of the extra machines.
If you have a business and a business server, you can send the images to a server without having to get the extra drives. This is actually quicker and more efficient. With the server, you can image multiple machines (known as multicasting) at a time. For example: at the school where I worked, we would image a whole computer lab at a time instead of duplicating them manually.The nice thing was you could automate this to be done late at night when the computers in the labs were not being used.
Did not draw all the cables, but you get the idea. You can also make additional updated images to the server. So if a hard disk dies, you can simply re-image the replacement drive and be up and running again in no time. Actually today larger businesses are going disk-less with virtual machines via ltsp, aoe, and iscsi, but that is another discussion.