First of all, you need to have two files on your hard drive: the iTunes patch, the Apple's iTunes installer disk image. (See picture) If you don't have these two files, you have to download them.

You can download the iTunes patch here, and Apple's iTunes installer image here.

The iTunes installer is inside the disk image, where the iTunes patch cannot change it. So you have to:

1. Mount the disk image “iTunes.smi” by double-clicking on it. You will see a new “virtual disk” icon appear on your desktop:
   
   
2. Open this “virtual disk”. Inside you will find an icon labeled “Install iTunes”, which you must copy to your hard drive.
   
   
3. Now in your hard drive you must see these three icons:
   

Launch the iTunes Mac OS 8 patch, and from the options select “Patch the iTunes Installer” (whether you select iTunes 1.0 or 1.1 depends on which version of iTunes you have.)

Select the disk that has the “Install iTunes” icon in it, and press the “Install” button to begin.

After the installation is complete, you will find a new icon on your hard drive:

Important: before you run the installer, make sure that you have a folder named “Documents” in the top level of your hard drive. If you don't, create one. iTunes will not work without it.

Double-click the icon labeled “Install iTunes for Mac OS 8”, to install iTunes in your computer.

(Note that although Apple's installer includes the CD Authoring support files, it does not allow you to install them on systems older than 9.1. To install these files, you will later need to select the option “Install CD Authoring Support” from the iTunes Mac OS 8 patch installer. )

After you have completed the iTunes installation, you will find a folder named “iTunes” in your hard drive. If you have a folder named “Applications”, the “iTunes” folder will be in there.

Right now, the iTunes application is still locked so that it will only work on Mac OS 9. To remove the lock, run the iTunes Mac OS 8 patch installer one more time.

This time, select the options “Patch the iTunes Application”, and “Patch the iTunes Extension”. (Note that if you are using iTunes 1.0, there is no option to patch the extension, because it's already unlocked.)

After the installation is complete, the original (Mac OS 9 - locked) iTunes Extension will be moved to the Trash, and inside the iTunes application folder you will see a new icon labeled “iTunes for Mac OS 8” which is unlocked and will run on Mac OS 8.6. (You can delete the original “iTunes” application.)

At this point you will have to restart your computer to activate the iTunes extension.

If you encounter any error messages during the installation, check out the iTunes 8 patch FAQ.

Well, that's it. You are now ready to enjoy iTunes on your Mac.

P.S. Under Mac OS 8.6, iTunes online help is not accessible though the application for some reason. If you want to access it, you have to go to the Finder and select the “Help Center” item from the “Help” menu.