Launch the homebrew channel either via bootmii or priiloader, then install a patched IOS60 into slots 70 and/or 80 (70 for 4.2 and 80 for 4.3, but it's safe to just install both) (if you can't access priiloader or bootmii then your probably screwed)
You can download these from ModMii Classic's download page 1 IIRC. Enter "70" and "80" to toggle them for install, then install them using any wad manager (mmm, yawmm, wii mod lite, etc)