Lambert 's comment (Dedoimedo page) gives you the basics that you need to understand the process of making the Windows partition smaller, and creating a new partition for CentOS in the space newly made available.
As for the process of obtaining the actual system and installing it, then yes, you have it right. To summarize:
- Download the appropriate ISO image.
- Either burn it to a blank DVD, or prepare a USB stick from the contents of the ISO.
- Boot the computer from the DVD or USB stick, and follow the installer program's instructions.
Since this may be the first time you do this, I would suggest going with the blank DVD instead of the USB, it may be more simple if the target computer has a DVD drive available to it. Many DVD burning programs are available to do this. Just make sure you are burning the ISO file as a disk image, and not adding it to a new DVD file structure. Look for an option to "burn an image" in the software you use.