Most of the videos that appear to display GCs running copied GD-ROMs (phyical discs that have been burned) are believed to be fakes. I am however aware of a team working on a GC Mod Chip (nowhere near completion yet) but they have no released nothing more than images showing their hardware setups to reverse engineer all the required systems.
To answer your question, you currently cannot do it cheaply or without LOTs of effort, time and extensive hardware knowledge. The methods are not publicly available.
!NOTE! I will not post anymore information on this topic as it borders many legal issues and is against my personal morals !NOTE!
I do NOT condone these actions or even particularly like them, I only know of their existance and feel that the "Homebrew" excuse for such actions is unjust as it will only lead to piracy, which is for...
I'll stop before i say something i regret!
(MODs, please edit or delete my post if it causes any problems)
Regards,
Fox