Pre-setup:
Before installing WiiFlow your Wii or Wii U vWii must be soft-modded and have d2x installed in slot 249 with base 56 and slot 250 with base 57. If it's not soft-modded then go to CompleteSoftmodGuide to mod your Wii or Wii U vWii and then install the proper cIOS per instructions below.

For Wii:

• Download d2x-v10-beta52 for slot 249 and d2x-v10-beta53alt for slot 250 (d2x-v8-final still works fine as well). Note they are zip files that need the extension to be renamed to .7z because they are actually 7-zip files.
  
• Follow the guide here to use either ModMii or the official d2x-cios-installer v3.1 to install them.
  
• Tip: d2x-v10-beta52 has higher compatibility for NAND emulation and online stuff and less random crashes during game-play that’s why it’s in slot 249 while d2x-v10-beta53 has 2 USB ports support and fully plug & play USB devices, meaning that you can connect them when the game asks for them.
  

Installing WiiFlow:
Now that you have your Wii soft-modded and d2x installed:

• Download the latest official WiiFlow - WiiFlow 4.2.1. Note: This version does not support the latest Nintendont. Get WiiFlow Mastermod by Abz for Nintendont support.
  
• Extract it to your SD card or USB HDD. It should automatically put WiiFlow boot.dol, icon.png, and meta.xml in the /apps/wiiflow folder.
• Put your Wii game backups in the usb:/wbfs folder using Wii Backup Manageror a similar program. Or if you don't have any backups yet you can make backups with WiiFlow. More on that later.
  

Installing a optional forwarder channel:
Without this you must launch WiiFlow from the Homebrew Channel.
To install a forwarder channel to launch WiiFlow from the System Menu do the following.
For Wii:

• Use Wad Manager 1.7 or another wad manager app to install a WiiFlow forwarder channel. Note: You will need IOS 58 installed to use these v14b forwarders. Use ModMii to download and install IOS 58. Or you can use IOS 58 Installer provided your Wii has WiFi.
  

For Wii U:

• Use wiiflow channel installer v1.1 to install the default forwarder.

Launching WiiFlow the first time:
Start WiiFlow with your new WiiFlow forwarder channel or the Homebrew Channel. WiiFlow will create the necessary folders and config files and then display your Wii backup games. If you don't have any games in your 'wbfs' folder or there is no folder then WiiFlow will display the following message -
'Welcome to WiiFlow. I have not found any games. Click Install to install games, or Select partition to select your partition type.'.
If you know you have a 'wbfs' folder with games in it then follow the second choice to choose the partition (SD, USB1, USB2, etc...) your games are on. Otherwise insert a Wii game disc and select the first option to install it.
Adding Covers:
Now you should have at least one game case showing (maybe multiple of times) but it's blank. We need to cover it. Move your pointer to the bottom of the screen and some icons should appear. Choose the one that looks like some gears. This is the config icon and it will take you to the Main Settings menu. Click on the first option which should be Download Covers. This takes you to a new menu with more options. The only two we need to worry about now are the first two - Download All or Download Missing. The other options will be explained later. To Download all covers click 'All'. Later on if you install some more games you can just choose 'Missing'. When it's done click on 'BACK' to return to the main screen. Of Course if your Wii isn't connected to the Internet then this won't work for you. You will have to download the covers on your PC and transfer them to the appropriate WiiFlow covers folder. You can use Wii Backup Manager with your USB HDD connected to your PC to download the covers for you.
Launching a game:
This should have you set up to play Wii games from your USB HDD. Browse through your games with your wiimote and press 'A' while pointing at the game you want to play. And then click on the 'PLAY' button to launch the game.