Go to mac os 9 lives forums they have two OS9 installs. Try to just copy them to your HDD, then "unzip" them (they are .dmg or .sit usually). Then to try, if it works, open an OS9 application or game. You should now see OS9 opening/booting in a smaller window.
If that does not happen, after trying the above. Then go to disc utility. There you can see a small checkbox that says "install OS9 drivers". You need to check this when you format the drive. Well, formatting the drive will delete your OS X install, which you don't want.
Solution: download the app Super Duper by shirtpocket software. Connect an external HDD or USB drive (external HDD with Firewire connection preferred) and use SuperDuper to make a bootable backup on your external HDD/USB.
Try booting form the external backup to be sure, everything worked out fine. Now open Disk Utility while being booted into your backup and format the internal HDD HFS+ with Apple Partitition Map and checking "install OS 9 drivers".
After formatting, use SuperDuper to get OS X back from your backup to your internal HDD.
Now try installing and using OS 9 again.
Note: be aware OS9 in classic mode will not be super fluently with heavy apps or with games. Office apps will work fine though.
If your Mac is a Mac that was able to boot into OS9 anyway, then use OS9 rather directly, instead of inside the classic environment.