Disclaimer: there is a chance that you may mess up your phone. I am warning you ahead of time that you are doing this at your own risk and that I might not be able to help you!
I didn’t bother backing up… you may want to do so :
1. You will need Android SDK installed (follow the instructions : http://developer.android.com/sdk/index.html )
2. install heimdall ( https://developer.mozilla.org/en-US/docs/Mozilla/Boot_to_Gecko/B2G_build_prerequisites#Installing_heimdall )
3. download Hardcore-speedmod ( http://people.mozilla.com/~nhirata/cynogenmod_galaxy_s/Hardcore-speedmod-i9000.zip ) and unzip it
4. attach the device to the computer, and hold down volume + home button to get to the download
5. You can use some sort of UI for heimdall, I used the command line :
heimdall flash –kernel zImage-i9000
[props to : http://www.mydaily-gadget.com/how-to/9487-install-clockworkmod-recovery-samsung-galaxy-s-heimdall-suite ]
6. once it downloads, reboot the phone if it hasn’t already and hold both volume buttons + power button while still plugged in.
7. once clockwork mod launches, it will allow you to adb into the device. make sure you have write access to the sdcard by adb shell and then doing a ls sdcard.
If you don’t have write access, chances are that the sdcard is not mounted, and you will have to go through the clockwork mod options to mount the sdcard.
8. download the latest nightly cynogenmod 10 for the galaxy s : http://www.cyanogenmod.com/devices/samsung-galaxy-s
9. push the zip file to the sdcard and rename it update.zip
10. using the clockwork mod select the file to use as the update.
11. the phone will reboot near the beginning of the update process. don’t panic. The clockwork mod just got updated in the process.
12. wait til the phone reboots again, and you should be in cynogenmod 10 nightly.
[Note: if you find there needs to be steps added or what not, please feel free to comment]