Install GCOS 1.5 and autoboot (no menu) with Action Replay

You'll need:
-an official Nintendo Memory Card, 251 or 59.
-a way to run dol files on your gamecube, like PSO1&2+BBA
-Action Replay for Gamecube

1) ** This step will erase everything on your memory card **
   Insert your Official Nintendo Memory card into Slot B.
   Run "1 format.dol", press A to format it.

2) Reset your Gamecube and go into the GC bios save manager, 
   it should say you need to format, so format it.

3) Run "2 install-ARcodes.dol", press X to write the file, 
   and press R to select slot B.

4) Run "3 install-level2.dol", press X to write the file, 
   and press R to select slot B.

5) Run "4 install-gcos15.dol", press X to write the file,
   and press R to select slot B.

6) Put in Action Replay, reset, go to Start Game, open the lid,
   close the lid, and GCOS 1.5 should boot up. Enjoy!

~DeepBlade