Go to Bannerbomb. Read the instructions and follow them. In brief, you download and unzip Bannerbomb onto an SD card. Then you download the Hackmii Installer and unzip it, copying installer.elf to the card’s root directory and renaming it to boot.elf.
Note that the bannerbomb site offers a few alternate versions of the software. If the main version doesn't work for you, go back and try the others one by one until you find one that works on your Wii.