Well, you'll need the homebrew channel, but that's hard to install on 4.3, and if you're asking that question, I assume you already have it. If not, you can search on YouTube for how to install it on 4.3, there are a few tutorials.
Then, I'd recommend making a NAND backup in case something goes wrong. After that take out all your Gamecube hardware (memory cards and such), as they can be corrupted while you're installing and uninstalling IOSes. Then you'll need to install DOPMii (link in the source), go into it, select an IOS (usually 36), go into the IOS Menu, scroll to the stubbed IOS, and hit the minus button. That uninstalls the stubbed IOS.
For CIOS, just follow the tutorial in the sources.