How Do You Use a .iso file?

You need a program that will burn it to a disc.

You need to use a program that can burn the iso and not just copy the file to a CD. Most decent CD writers have a way to burn a CD image file. I think an ISO file contains the lead in and lead out for the CD so you are basically copying all the bytes directly to the CD. By copying the file instead, the program creates the lead in and lead out so the ISO is unusable.

using .iso file
so that's it? you just burn it to a disc and it will work on the computer?

Yep, But you have to burn it as an image not as a file.

Also, device emulator programs (Daemon Tools is one example) allow you to create a virtual drive on your computer, so you can load the .iso file that is already sitting on your hard drive and treat it as if it were actually on a CD/DVD. You could do that instead of burning it onto a CD/DVD media.

After installing the drive emulation program, you just fire it up, mount the image file (e.g., the .iso file) -- assigning it a drive letter (ex., "E"), and go from there, treating the mounted image as a CD/DVD drive.

If that's what you want to do. That's another way to "use" an .iso file.