I don't remember if there is a dedicated thread for InstantCake, but here are some instructions I found through Google.
If you can't get it to work, I found this post. It's from 2009 so I don't know if it still applies.
Because InstantCake can be flaky, you might be better off getting an image for your model Tivo here
and using either WinMFS (for .tbk images) or the MFS Live CD (for .bak images) to create the new hard drive.
WinMFS and the MFS Live CD can be found here:
and help using either can be had over in the TiVo Upgrade Center.