If you don't have priiloader yet, get it first. Especially if that bootmii is installed as an IOS.
From modmii, you may want to get "cIOS 236" (aka: a patched IOS36) and the latest version of d2x. 236 is probably not needed (249 can most likely also be used to install custom wads), but it won't hurt.
Also get postloader or neogamma as loaders. And MMM, dop-mii or wad manager to install wads (one of these will probably be advised by modmii anyway).
I'd say the above are the prerequisites. Both postloader and neogamma can be used for wii games as well, so I suggest you try to get that working first. At the very least to rule out problems with the loader first.
Once all that is okay...then it's just following the guide.
Aw, crap...just now saw the part of sneek. Okay...let's start over.
Start by answering 3 questions:
1) do you have bootmii as IOS or as boot2? (this is important)
2) what flavor of sneek do you want to use? Basically, it's 2 yes/no questions:
2A. do you want your emulated nand on the USB drive (as opposed to on the SD card)?
2B. do you want to run games from disc or from the USB drive?
I would suggest to go for the USB option in case A, as it would otherwise clump up space that could be occupied by gamecube games.
...and neek2o for the overall. But that's for later.
3) do you want to use your own current wii's nand, or start with a completely new one? (if the former: start by creating a backup with bootmii).
That's it for now. I'll respond later when you give some answers.