In 10.4.8... you don't.
The hdik tool, which you need to make a RAM disk, won't work without the rest of the OS being loaded AFAICT. But to get the rest of the OS running from a boot CD, you need a RAM disk. Catch-22.
Even Apple's own System Image Utility didn't work on 10.4.8 the last time I tried it. That should tell you something.
Maybe this situation will improve in Leopard. Then again, maybe not.