Is there a guide around on how to do this, like where the solder points are and what not? I have a Cthulu so I’m assuming I’ll need to order an Imp Board right?
Yes you’ll need an imp or dpdt switch.
Here’s a decent guide: http://shoryuken.com/f177/ps3-te-stick-modded-360-sf4-madcatz-fightpad-56k-death-196633/
site:shoryuken.com madcatz fightpad - Google Search second result.
You should really look up the “Dual Strike SMD” ps3 board from SRK user “Gummowned” It is a small board that should make dual modding your fight pad a lot easier. There is no easy way you are going to fit a Cthulhu inside a madcatz fightpad.