Download and run the official ModMii Installer.
The installer is self-explanatory, but if for some reason you run into issues you can download the latest zip from HERE and unzip it anywhere on your PC. Then just double-click ModMii.exe or ModMiiSkin.exe.
You can use the ModMii Wizard to softmod any Wii. Even if the Wii is already softmodded you can update all your softmods (overwriting existing softmods) by telling ModMii that it is the first time you're modding your Wii, even though its not. You can also use this wizard to setup up a USB-Loader instead of having to later use the USB-Loader Setup Guide
This wizard allows you to enjoy many of the benefits of a softmodded Wii without installing any unofficial content (ie. should not void warranty). It will allow you to boot an emulated nand off external storage instead of the internal nand memory (i.e. initiate an exploit to launch Casper which redirects to load an emulated NAND).
On a related note, did you know you can make a nand backup on a virgin wii before even installing the homebrew channel or bootmii? Inside ModMii's support folder is a file called 'Casper2BootMii.bat', run it and ModMIi will save Casper and bootmii files to the right places so that you can make a nand backup directly from an exploit.
Set up a USB-Loader to saveplay backups of your games tofrom an external hard drive.
You should only need this particular wizard to correct problems caused by inferior and outdated softmods like 'cIOScorp' or 'DarkCorp'.
The HackMii Installer can't run properly on Wiis that have been overly or improperly modified. If the HackMii installer freezes, or your Homebrew Channel is upside-down or you get a no vulnerable IOS error, this guide will resolve the issue.
For those interested in the specifics, this wizard will instruct you to launch a WAD Manager via an exploit and install some clean IOSs that the HackMii Installer likes (e.g. IOS58).
Syscheck is a homebrew application you can run on wii's that have already been modified. As its name implies, it will CHECK your SYStem and save a report to your SD card. If you open your sysCheck.csv log with ModMii it will analyze your setup and identify which mods are out of date. ModMii will prompt you to download only the files required to update your softmod and build a custom guide for you to follow.
You can analyze a log by dragging it onto ModMii.exe or a shortcut for it - or you can launch the syscheck updater from ModMii's main menu and browse for your log like a caveman ;)

Send cmd line instructions to ModMii to perform various instructions. To see the help menu just open a cmd window where ModMii.exe is saved and type 'ModMii Help' then press enter.
This is actually how ModMiiSkin works. You could write your own apps that call ModMii to do the heavy lifting! Just please remember to give credit, and heck you can email or message me on gbatemp if you'd like to collaborate!
Remove unnecessary files from your SD Card after you're done softmodding your Wii.
Instead of having your wii readwrite to the internal memory you can have it use external storage. That's like being able to make a backup of your Wii and load it on a friend's console!
Use ModMii to assist you in setting up and installing neek or neek2o which are SD/USB NAND/ES emulation kits for the Wii. This allows you to load an emulated NAND from an SD Card or a FAT32 formatted USB Hard Drive. This means you can carry a backup of personal system menu with all your game saves with you and load it on any Wii. If using neek+di/neek2o+di you can even load games from your external Hard Drive in the disc-channel. An emulated NAND is required to load neek/neek2o, but ModMii can help you with that too!
Build an emulated NAND that you can load using neek/neek2o or other tools like mighty channels or triiforce. ModMii allows you to choose from various options when customizing your emulated NAND.
Modify an existing emulated NAND. You can installuninstall Priiloader, install a custom system menu theme and more.
neek2o+di supports loading Wii game saved as WBFS files (most common format), but the original neek requires games be extracted to fst format (aka SNEEK Format). To accomplish this you can use ModMii to extract a whole folder of games to fst format.
Why does this even exist you ask? Two reasons - first because it's grandfathered to support the original neek... but that's lame because you use neek2o right which supports WBFS files which is pretty much the gold standard at this point. Well the second reason because the FST file system by design allows you to see all the individual files that make up a game. So if you want to make some custom levels or tweaks this file system makes it super easy to swap in new files.
A forwarder is like a shortcut that starts an application, channel or website.
To build a forwarder, from ModMii classic's main menu, enter 'A' for advanced downloads, then enter 'F' for Forwarder DOLISO Builder.
Forwarders can be run using the Homebrew Channel, or injected into system menu channels, or installed via Priiloader so you can control what your Wii boots when it first starts up, etc. You can even inject your forwarder into an ISO (which is basically a 'game' you can load from a USB Loader to start a different application).
Using ModMii You can make forwarders that launch an app (dol or elf file) from an SD Card or a USB Hard Drive, or launch a channel you already have installed on the Wii, or launch the internet channel to a specific webpage.
If you want to inject your forwarder into another channelWAD, download Customize Mii from ModMii's download page 2 and use it to load a channelWAD, and inject your new forwarder.dol that you build with ModMii.
Only for advanced Wii hackers. Only use this function if you know what you're doing.
From ModMii classic's main menu, enter 'A' for advanced downloads. Enter the number corresponding to the IOS or cIOS you would like to modify. You can enter 'BETA' if you want to build a different version of the famous d2x cIOS, but the version bundled with ModMii is all you should ever need.
Then ModMii will prompt enter a new slot number, or version number or apply patches.
Only for advanced Wii hackers. Only use this function if you know what you're doing.
From ModMii classic's main menu, enter 'A' for advanced downloads. Then enter 'U' for User-Defined Custom Download. From here you can download and optionally patch an IOSMIOSSystem Menu that isn't listed on ModMii's standard download pages.
Basically this is the same as NUS Downloader but can make IOS patches without having to use a second application.
Only for advanced Wii hackers. Only use this function if you know what you're doing and are able to tolerate and report potential bugs in the beta cIOS.
From ModMii classic's main menu, enter 'A' for advanced downloads or '4' for Download Page 4. Then enter 'BETA' to build a different version of the famous d2x cIOS, but the version bundled with ModMii is all you should ever need. Under Contruction.