Use Modmii.
It downloads everything you need to where you need it, generates a custom guide for you to follow, and overall pretty much everyone loves it!
As already stated, the only way for Modmii to mess something up is if the power cuts while installing something to the Wii. Even if you quit Modmii in the middle of a download, it verifies all files, so there's 0% chance of installing a bad file.