First of all : have a USB hdd
no, really it should be as easy as :
1. download the homebrew and read the readme.txt (if any, or search for the official website help page) which explain how to install it.
It's usually always "copy boot.dol, meta.xml and icon.png into SD:/apps/name_of_the_homebrew/"
2. Read the readme to learn where to place your ROMs or ISO on the HDD.
Often, you can place them where you want.
But some emulators may require specific folders (for example WiiSX need a bios folder).
3. connect your HDD to USB Port 0 (the one near the edge)
4. place your SD card in the Wii.
5. launch the homebrew using the Homebrew channel.
6. follow the visual instructions/menus to load from USB.
It's really not complicated.
Be sure that your wii is hacked with the latest files (try modmii's syscheck analyze to tell you if you are up to date).
Also, did you followed the tutorial on this page ?
http://wiki.gbatemp.net/wiki/WiiSX_compatibility_list_%28beta_2%29
There's a "help" section which explain what you have to do and where to place the files.
I recommand you use WiiSX beta2.1 mod02 or mod03.
Don't use Mod04, it's a little slower to read games from USB.
