For versions 3.4 (Hack without game disk)

1. Get your Wii's mac address:The MAC address of the Wii can be found by:
From the Wii Channel menu, select "Wii Settings" (the round button on the bottom-left of the screen with "Wii" on it)
Select "Internet," then "Console Settings."
The MAC address of the Wii console is displayed on the first line.
2. Format your SD card. Fat32
Panasonic SDFormat preferred. Dont skip this step.
3. Download the HackMii installer
You can use the mac address to build a custom hackmii installer at LetterBombhttp://please.hackmii.com/
MAKE SURE YOU SELECT YOUR REGION up top
If you get the captcha wrong, it will RESET your region. This happened to me. If you do, the installer, well, in my case hung the wii. Had to hard boot it.
4. Copy downloaded files to root directory of SD card.
I used windows xp.
5. Put card in wii and start it.
6. Hit the envelope in the lower right hand corner
If you dont see the red message with the bomb, hit the - or + buttons to choose a different date.
7. Click on the big red envelope with the bomb.
The hackmii installer launches. (note if it hangs, you might not have done step 2, or you might have chosen the incorrect region)

Part 2 :
Installing the Homebrew channel and BootMii

Read the Scam warning and press 1
It may take a minute or two for the "press 1" to appear- be patient
Install the HomeBrew Channel
Install Bootmii as Boot2(boot2 may not be available, IOS installs automatically)
Prepare the SD-card (By reformatting it)

Part 3 :
Make a Backup (Not Optional!)

Load HBC, press -HOME-, launch Bootmii
Make a nand backup
Press Power, Power, Power, Reset, Reset
Once the backup is complete, exit bootmii to HBC
Backup your Bootmii files to your PC
sd:\bootmii
sd:\nand.bin
sd:\key.bin

*DO NOT LOSE THESE FILES*

Now you have your Homebrew channel installed!